Las Vegas
It's been a couple years now since Tasha's infamous Hotel Security List has been updated. The following was posted on the board here back in December 2010. Obviously, things change; so if anyone has legitimate current info, please post it here and I'll see if I can put together a 2013 version.
The M Resort- No security.
Four Seasons- Security at the elevators after 6pm, sometimes they are always there, but very nice.
The Hotel- No Security
Mandalay Bay- No Security
The Luxor- No Security but elevator requires a key to access a floor. East and West wing towers do as well now.
The Excalibur- Security and sometimes very rude.
The Tropicana- No Security
Hooters- No Security
The MGM- No Security at the main elevators.
The Signature- Have to pass a guard at a gate so need your (real) name called down
The Mansions at the MGM- Key Entry only
New York New York- No Security but some towers require a key to access a floor.
Monte Carlo- No Security
The Bellagio- Security but very nice
Planet Hollywood- No Security.
The Paris- No Security
Bally's- No Security
The Westin- No Security
The Platinum- No Security
Hilton- NO Security.
Ceasers Palace- Security at some towers but usually nice.
Bill's Gambling Hall- Not sure about now but when it was Barbary Coast, they would try to take pictures and have been known to chase you to get one, not kidding.
Imperial Palace- No Security
Harrah's- Security and very rude and ask for ID's however there is a secret back way up that isn't that hard to find and by passes security. Also if you see the lady before 8pm, she can access the elevators near the parking garage entrance. If she goes to the 4th floor, she can get to any tower 
The Flamingo- No Security
The Mirage- Security after 9pm or 10 but always sweet.
T.I.- Sometimes they have Security and other times not.
The Sahara- No Security
The Stratosphere- No Security
The Wynn- Security after 9pm but usually no problem.
Encore- Security, need to show them room key.
The Palms- Security all the time. Sometimes they are accommodating and nice, other times rude and questionable.
Palms Place- Need a key card.
The Rio- No Security for the most part but sometimes they do, usually nice but can be judgmental so best to meet downstairs.
The Orleans- Security but nice.
The Gold Coast- No Security to my knowledge.
Venetian- Security after 9pm on the 2nd story but requires key to enter and always security on main level.
The Palazzo- Security
Circus Circus- No Security
Riviera- Security at some towers but usually nice.
Sun Coast- No Security
South Point- Security but never a problem
GVR- No Security
Golden Nugget- Security after 9pm but sweet
Wild Wild West- No Security except patrolling the parking lots which is how you enter the room.
Palace Station- No security to my knowledge.
Trump Tower- Need a key card to access elevators.
Red Rock- No security but key access only.
Aliante Station- Not to sure, I have yet to go there.
JW Marriot in Summerlin- No security.
New HRH tower- Security
Aria- Security after 5pm, best to just meet downstairs IMO.
Mandarin Oriental- Security after 5pm, best to just meet downstairs.
Vdara- Security after 5pm, best to just meet downstairs.
New tower at Plant Hollywood- Not sure, if anyone knows, please let me know.
Hotel 32- Need key to access the elevators that go to the floors.
Tuscany Suites Las Vegas- Need a keycard to access the halls.
Marriot Grand Chateau- Must have keycard after 7pm.
Cosmopolitan opening tonight not sure about security.
Downtown hotels excluding the Golden Nugget I do not visit, so have no knowledge about.
Thanks for the update Des, here are a couple more:
Vdara now has 24 hour security as well as Aria. Just show a room key. They don't care who you bring with you. At the Vdara a couple of them might even give you a high five. 
The M Resort- No security.
Four Seasons- Security at the elevators after 6pm, sometimes they are always there, but very nice.
The Hotel- No Security
Mandalay Bay- No Security
The Luxor- No Security but elevator requires a key to access a floor. East and West wing towers do as well now.
The Excalibur- Security and sometimes very rude.
The Tropicana- No Security
Hooters- No Security
The MGM- No Security at the main elevators.
The Signature- Have to pass a guard at a gate so need your (real) name called down
The Mansions at the MGM- Key Entry only
New York New York- No Security but some towers require a key to access a floor.
Monte Carlo- No Security
The Bellagio- Security but very nice
Planet Hollywood- No Security.
The Paris- No Security
Bally's- No Security
The Westin- No Security
The Platinum- No Security
Hilton- NO Security.
Ceasers Palace- Security at some towers but usually nice.
Bill's Gambling Hall- Not sure about now but when it was Barbary Coast, they would try to take pictures and have been known to chase you to get one, not kidding.
Imperial Palace- No Security
Harrah's- Security and very rude and ask for ID's however there is a secret back way up that isn't that hard to find and by passes security. Also if you see the lady before 8pm, she can access the elevators near the parking garage entrance. If she goes to the 4th floor, she can get to any tower
The Flamingo- No Security
The Mirage- Security after 9pm or 10 but always sweet.
T.I.- Sometimes they have Security and other times not.
The Sahara- No Security
The Stratosphere- No Security
The Wynn- Security after 9pm but usually no problem.
Encore- Security, need to show them room key.
The Palms- Security all the time. Sometimes they are accommodating and nice, other times rude and questionable.
Palms Place- Need a key card.
The Rio- No Security for the most part but sometimes they do, usually nice but can be judgmental so best to meet downstairs.
The Orleans- Security but nice.
The Gold Coast- No Security to my knowledge.
Venetian- Security after 9pm on the 2nd story but requires key to enter and always security on main level.
The Palazzo- Security
Circus Circus- No Security
Riviera- Security at some towers but usually nice.
Sun Coast- No Security
South Point- Security but never a problem
GVR- No Security
Golden Nugget- Security after 9pm but sweet
Wild Wild West- No Security except patrolling the parking lots which is how you enter the room.
Palace Station- No security to my knowledge.
Trump Tower- Need a key card to access elevators.
Red Rock- No security but key access only.
Aliante Station- Not to sure, I have yet to go there.
JW Marriot in Summerlin- No security.
New HRH tower- Security
Aria- Security after 5pm, best to just meet downstairs IMO.
Mandarin Oriental- Security after 5pm, best to just meet downstairs.
Vdara- Security after 5pm, best to just meet downstairs.
New tower at Plant Hollywood- Not sure, if anyone knows, please let me know.
Hotel 32- Need key to access the elevators that go to the floors.
Tuscany Suites Las Vegas- Need a keycard to access the halls.
Marriot Grand Chateau- Must have keycard after 7pm.
Cosmopolitan opening tonight not sure about security.
Downtown hotels excluding the Golden Nugget I do not visit, so have no knowledge about.