Haven't seen anyone mention this yet, so I'll be the first.
The 280.26 drivers have been out since 8/9. Late last night I decided to try updating from 275.33. It was nearly a nightmare, but it could have been my own fault.
I downloaded the update and put boinc on suspend. Perhaps this was a no-no and maybe I should have just exited from Boinc, I don't know. Once the update was complete I was not able to connect to Boinc from the manager. Even a reboot did not help. Boinc manager just sat there trying to connect forever. Occasionally, I would click the 'cancel' button on the dialog popup box and it would show my tasks, but the project tab was empty. The tasks were not crunching and I could verify it with the task manager. I tried reinstalling Boinc, but that did nothing. I then reinstalled 275.33. Strangely the problem still persisted. I thought grimly that I may have borked all of my tasks, including a 100+ hour PG SOB task. If you could have seen my face at that moment you would have felt pity. But, I decided to try random things and killed the ever present Freehal processes and suddenly Boinc started working.
The moral of this story is that I don't know what went wrong. Perhaps it was because I suspended Boinc during the upgrade. Maybe it was Freehal that was the problem. Either way, I went back to running with 275.33. I may even try to upgrade again. I'm curious if anyone else has tried to upgrade.