Much like the other post regarding this problem, I found that I couldn't engage the safety button to release the shifter and place it in gear from park. I, however, had bought the car in January and it didn't start doing this until warm days. The dealership told me the previous owner had spilled cola in there and that it was gumming up the works. I find this to be ridiculous, because it would always feel like there is sludge in it. I agree with Millerok in that it seems to be related to the brakes, since they go all the way to the floor, vibrate as if the ABS is kicking in, and then it works perfectly fine. I have made an appointment at another dealership and will keep you posted.

Update from Jan 26, 2010: After bringing it to 2 separate dealerships, they found the problem. I had bought the car used and apparently the owner before me had spilled coffee or soda down the shifter. They said this was gumming up the system somehow and that it had to be completely replaced. It would have cost about $700 for the parts, but it was under warranty. For anyone else having this problem, though, there is a solenoid in the shifter that disengages when the brake pedal is depressed, making it so you can push in the button. This solenoid, like any other one, can go bad. If you hear a clicking when you push the brake in, it's probably what that is. I believe that is a relatively easy fix and not that expensive.

Update from Feb 3, 2012: Finally, after having the shifter changed out twice with no improvement, I pushed to have the brake switch changed. The brake switch is under the brake pedal. When the pedal is depressed, the switch is engaged and it tells the solenoid in the center console to release the button on the shifter. Since I was having problems with the brake lights and the shifter at the same time (which were both intermittent issues that seemed to happen at the same time), I figured the only common denominator is that switch. I had it replaced and haven't had a single problem since.