That is why I try to warn people about DSLRs. It is a disease to constantly upgrade your equipment!
I made the mistake of getting a Canon 70-200 f/4L IS. The build quality of this lens just sets it apart from my other equipment. Very quick in auto focus. No hunting. Very quiet. Now, I have lens envy.
Now, I have ordered a 40d, which I think is the best bargain out there right now. I was sorely tempted by the 50d, but I didn't think the extra features were worth the extra money. The microadjust made me think about it, however.