When I upgraded from a Rev. A 12" PowerBook to a Rev. D 12" PowerBook, the biggest noticeable difference was the jump in disk performance.
The Rev. A had a 4200rpm drive, the Rev. D a 5400rpm. If you're at all concerned about performance, I'd wait until you can get your hands on one and test it, as the MacBook Air has a 4200rpm drive. As edesignuk said, disk performance is typically the biggest bottleneck, depending on task (because the only mechanical device in the whole machine is the disk, and motors and arms are orders of magnitude slower than electrons). I also agree with the consensus here that upgrading by 0.2GHz is a waste of money - buy a nice neoprene sleeve or an external BT mouse instead, and save the change (for the next keynote...)
AppleMatt