I had this problem, took my car up right up to the dealer- I hate lit-up warning lights on my dash. It took a bit to fix, basically the computer suddenly decided that all tires needed 70-something psi, so no matter what the tires had they were going to come up short. The service guy had to enter the correct settings into the computer for each wheel individually though- he couldn't reset them all at once. An hour and a half later I was out of there, and one month later everything is still ok...
-V