I just came across this recently.
If you haven’t already found the solution, it seems that it’s a conflict between FCP 4.5 (which I’m guessing you’ve got) and Quicktime 7.3.
The solution – which i found on some apple discussion boards, seems to be to reinstall quicktime 7.2 as well as FCP. Apparently quicktime gets embedded in FCP file structure – or something. Some other ppl were mentioning that an OS reinstall might be necessary too.
Anyway, one workaround I’ve used is to capture with Quicktime Pro. Its obviously limited w/ no Batch Capture etc, but it works.
Simon