This post is fairly old, but as there are still a few new '09 450's for sale out there and the 2011 is virtually the same as the '10 I thought I would go ahead and chime in on the subject...also, I know there are plenty of riders out there on 06-09 YZ's that are considering a new bike so this thread may still be of some interest.
First off, if you currently own an older YZ450 in good working condition or are looking to purchase a new bike and know of an 06-09 YZ that has been well cared for (or is a dealer left-over), don't hesitate to pick one up up or throw a few bucks towards freshening up your older model. I've owned both the 09 and '11 YZs, and while there are certain aspects of the newer model that improve upon the older bikes, as a whole I would have to say that I actually prefer my '09 to the '11, especially for trailriding. If you're a full-time motocrosser, or just absolutely have to have the latest and "greatest" thing out there, than the new YZ may be the bike for you...if not, save yourself a boat-load of bucks and stick with the older models until Yamaha
comes out with the next iteration of the new-generation 450.