Currently, you do not need a passport to return to the States from TJ.  However, starting Jan 2007, you will.  It's difficult to believe that the Feds will stand by that decision because it will literally decimate the tourism trade in TJ and Mexico will howl loudly.  You may say to yourself, "So what? Who cares what Mexico thinks?"  But know you this: Americans cross the border and buy blankets and velvet pictures of Elvis; Mexicans cross the border and buy refridgerators and washing machines.

In any case, I've already renewed my passport because if the Feds do go through with their threat, the State Department will literally be inundated with thousands and thousands of passport applications.

Not yet, but you will after December 31, 2006,,get one now as their will be a rush near the end of the year in southern California!!

If you have a passport, it won't hurt to bring it along even now. When it comes to the border and the Office of Homeland Security, they are so fucked up, they keep changing their minds all the time.

I have heard a few instances where fucking jerk off customs/border agents have harrassed people coming back to the US, asking for their passport.
BUT keep in mind these instances were when a person was beligerent or drunk.

If you're courteous and respectful, listen to the guys checking your ID's, you will not have any problems.

As I've said in the past, if you're going to TJ, it's best if you arm yourself with the best and most current info.
Join a TJ pay site.
They have been chatted about on this discussion site, you just need to do a little bit of reading.
 
There are some free ones out there but they only have basic info.
The pay sites have a lot more detailed info.

Currently No.  But brining it is  a good habit to develop.

There is a lot of opposition to requiring a  passport to travel to Mexico. This idea may die a slow death. There's also talk of a special border crossing card for the US/Canada and US/Mexico crossing.

My understanding is the deadline has been pushed back to 2008.  Contrary to what appears on the CBP website.  

This has been changed several times, but at this moment you will need a passport or a "pass card"(being developed) by July of 2009.