For sports games with annual releases, I don't think it would be unusual for a game to not sell very well in retail 2-4 months before the new version releases. Aside from gamers waiting for the bargain bin (2009 is $30 now brand new and even cheaper used), not many gamers are going to bite on 2009 right now when 2010 is just around the corner. On the NHL 10 forums online, gamers are advising those interested in buying it to wait off on NHL 10 and go with NHL 11 instead because buying an NHL game in March at full price is "too late" (even though it's six months away and playoffs haven't started yet). Annual sports games don't have much of a shelf life.
I suspect that 2010 will be quite popular when it releases. I get a sense that there isn't as much hype this time around though. Undisputed 2009 was hyped up a lot because it was the first UFC game since after the sport became very popular. The older UFC games were released before the UFC was mainstream. And gamers had high expectations for 2009 given how much THQ hyped the game's realism and what not. For 2010, I don't think the excitement will be the same. But it should still sell well and it'll be a very popular rental item this year just like it was last year (many gamers felt 2009 was too bare bones since you could easily beat career mode before your 5 day rental is up). UFC is a popular brand.
I'm more interested in seeing how EA Sports MMA will do. It doesn't have the UFC name but it does have some recognizable fighters (Randy Couture, Fedor, Dan Henderson, etc.) and the EA Sports brand behind it. Having played Fight Night Round 4 recently and loving how fluid the controls are (right stick total punch control) compared to more robotic stiff movements in Undisputed (mapping strikes to the buttons), I'm interested in seeing the right stick striking system used for MMA. Gameplay wise, I think EA Sports MMA will blow Undisputed out of the water this year provided that EA delivers the same quality they did with Fight Night. Eventually, I think the UFC is going to drop THQ and go with EA. THQ knows Pro Wrestling but they don't know sports (especially fight sports) like EA does. If UFC truly wants to grow as a brand, they'll need to go with EA eventually. Dana White loves to run his mouth about EA making a competing MMA game but as soon as UFC's contract with THQ expires, they're gonna jump in bed with EA I bet. EA may be the evil empire (well not as much as Activision and Ubisoft these days) but I have to admit that they know how to deliver quality.