I have had that happen several times. Go to Settings>Personal> accounts and sync, click your google account, and have it sync now for any and or all of those (google books, gmail, etc.). Now go back and the market usually works.
I don't about you guys. But I had this problem since build 1.8.19. I think that I have notice it more since I have spare batteries. Whenever I changed the battery out. The market doesn't sync. But I also had the problem even when I would reboot the rom. It becoming a real pain in the but. Just re-syncing doesn't solve the problem. I have tried a lot different things. Head over to the N1 page where I started a tread on this matter last week. I guess since it only affect a small amount it isn't a big deal. It suck that I have to keep switching back to build 1.8.12. If there is no fix I think that I will go back to CM.