I've been trying to set up Genius on iTunes 8 for the last few days, with no luck. I'm getting the "Genius results can't be updated right now. An unknown error occurred (4010)." message during Step 2, "Waiting for Apple to process information".
Looks like lots of other people are having this problem. What's so strange about it, is that it seems to come and go -- sometimes it works for people, sometimes it doesn't. Most people seem to think it's a server load issue.
None of us know if it's a server issue or not. But after the MobileMe fiasco, I'm starting to think that Apple is seriously unprepared in their "capacity planning" department.