Try going to the "Network" --> "Advanced" --> "802.1X" section of the system preferences and removing the user profile for your connection. Then set it up again.
Are there any other authentication profiles in the list?
Recently I had my late 2007 white MacBook serviced in an authorized reseller, to change the top cover, since my keyboard was failing in some keys.
Before that, everything was fine. But ever since then, my Airport connection at the office is failing. If I connect to my guest (unsecured) SSID, everything is fine. At home, with WPA Personal, also OK. The problem is in my production WPA Enterprise with 802.1X.
When I connect to it, it asks me for the password and then authenticates successfully. But, instead of getting an IP address, it simply starts the auth process again, and again, and again, in an endless loop. Both at the Console and in my authentication server, I can see "Authentication Successful" messages filling up my log. Eventually, it authenticates and stays connected, getting an IP address, and if this happens, everything works like a charm. But this is very intermittent.
I've set up a little SWF movie showing it happening, in case I was not clear (I'm not a native English speaker). It's uploaded to my website at: http://www.pistachecomcasca.com/movies/Airport_problem.swf
Any hints will be appreciated.