Check out this:
Especially the reset on the bottom line of the links (blue) click the triangle and follow the instructions for a soft reset, Ihad this issue recently, and it did drive nuts, I performed the soft reset and it has been fine since, if the soft does not help try the factory.
I had the same problem in that one MBP 10.6.8 would behave like this, other MBP no problem, iPad no problem. I did the reset from the problem MBP.
Hope this helps you.
It seems sometimes the MBP forgets it, don't know why.
Open airport utility Applications/Utilities/Airport utilitlty, it should find yours with a cryptic name, click continue, click "I want to change some settings for this airport extreme", click continue, click change these settings, enter a name, enter password, verify password, click continueclick thru to finish.
Hope this helps