Acura TL, 2002:
I changed the battery but couldn't reset the radio with the security code I had from when I bought the car. I read that there could be interference from the antenna and some one had suggested disconnecting the antenna as a solution, then enter the code. This was cumbersome and I could not remove all of the trim to get to the back of the radio. I read that some one drove down to an underground garage to get rid of all radio signals and then entered the code and it worked.
I was very skeptical, but had nothing to lose. I drove down to the 2nd underground level of our local hospital's parking garage and then entered the code. IT WORKED!
I don't know why it works, but if you're having this problem with an Acura, it definitely works. And, as long as you can get underground away from radio signals, it really works.
Hope this helps some one fix their radio.