Maybe he made the mistake of  registering it with the carrier with his real name. Even with tracphone you have to fill in your particulars when getting it activated online. If you want to remain anonymous, just don't put your real info on the form and use a computer to activate it that is not your own. I think they can track IP addresses.

All your calls, either from a normal cell phone or a disposable one, are recorded by the carriers and easily tracked in case LE needs to find you.
Disposable phones are only anonymous to us, regular folks - masks your identity but it doesn't erase it.

The real purpose of a hobby phone is to keep your personal phone free of tracks!  I believe the "Craigs List Killer" used a hobby phone & a common wireless connection.  LE tracked him to the building.  
I'm not going to murder anyone, so am not worried by this.  
skb

They didn't track "him" per se, they tracked the phone, and where it had been used, when it was used, and what numbers it called/received calls from.  They have no way of knowing who was holding it at the time those calls were made.

Disposable cell phones only work if you.. you know, DISPOSE of them

BTW in many countries, such phones are illegal.  I read a story a week ago about 26 million phones being turned off in Mexico because the law changed to make them illegal, and people were refusing to register.

The New York Times says this today:

"The phone number he gave three months ago was entered in a Customs and Border Protection agency database and came up Monday when investigators were checking the record of calls made to or from the prepaid cellular telephone used by the purchaser — at that point unidentified — of the vehicle used in the failed bombing, the official said."

The lesson is don't call any number that can be connected to you.

If you purchase a disposable phone, but then use it to call:
- your parents
- your brothers and sisters
- your friends
- your work colleagues
someone can figure out that it's YOU using the disposable phone, even though they didn't know you purchased it.

Moral of this story - use your hobby phone for nothing but hobbying! No other calls.

This is one more for a proposed TER Security Board; completely on topic.  There are a number of simple procedures and practices one can follow, some of them suggested in this thread, that can minimize the likelihood that your prepaid phone will be linked to you personally.  Among them, of course, is to not use your real identity when you register the phone or to establish call chain records of numbers you shouldn't be calling from your prepaid phone (like your home number or work number).

Could be lots of reasons and it's really dumb if you accept calls from people you know, especially if they are also on watch lists. A real burner is used to make contact once and only once for things not connected to what you are trying to hide.  Buy many, use once or twice if absolutely necessary and trash. Buy the cheapest call pack possible and wipe and burn. You also don't activate or leave voice mail. If you want to leave a really cold trail don't ever use them when you are at home or even close to home. New Yorkers should be feeling really good that this guy was dumb and only playing at being a radical. Of course, never use the same location twice to buy phones, better yet, pay someone to buy them for you; someone you don't know and who is not likely to see you again.