JIT on 2.1 ROMs is not the same as what's coming out on 2.2. JIT is what made cyanogens roms so good on the N1 as he'd optimized it for the Nexus One hardware; however unrooted N1 on 2.2 is averaging double the MFLOPs in Linpack than cyanogen mod was getting which is a pretty strong indication that Google's optimized it much more effectively.
Interestingly with all the iPhone OS talk, I think you should check out
this article, it's an excellent read and it details exactly how much JIT can do for Android in the versus battle with iPhone.
Also, I posted a list of some of the changes in Froyo that are 'less' documented here if you are interested. It appears that they are trying to increase the buttery smoothness of it to catch up to the iPhone.