Posting this in the RIGHT forum now.
OK, I have had this problem for the past day now. I'm using the HTC Droid Eris with the 2.1 update (the official one Verizon released). I've tried logging out and logging back in. That didn't work. I tried uninstalling the updates to Facebook since you can't actually uninstall it and using the prepackaged version that came with the 2.1 update. That didn't fix it. I tried reinstalling the updates, which didn't help. Then I tried logging out of the Facebook app, uninstalling the updates and clearing all the data in the Manage Applications area, reinstalling the updates, then logging back in. It seemed fixed for about half an hour, but now it's back to giving me "An error has occurred while fetching data. [102/Session key invalid or no longer valid]" on the news feed. The error for the notifications and mailbox mention both sections, but they're both the same error code 102. Any other ideas?