Most of Apple's updates are incremental. It's really only when there's a big jump that it's worth upgrading to the immediate replacements, such as when the Intel transition happened or when the G4 was replaced by the G5. The next big performance jump will happen early next year with the Sandy Bridge processors. Even then it'll be worth seeing benchmarks before deciding to upgrade from your current machine.I have the 09 i5 and honestly the new models don't offer anything worth upgrading too. While the SSD option is nice its priced a bit silly at this time.
I guess I will skip a generation or two.
For us lowly mortals on C2Ds of course it's a different story