I have not tried this myself, but I think the problem lies in some of the components not being carried over in Quicktime 7 after the upgrade. The mpeg2 component specifically does not seem to be carried over and may be the culprit in FCP's inability to capture HDV after upgrading to 10.6. As we know, MPEG2 is the compression used for all HDV.
This is an excerpt from a Creative Cow poster:
[[[ I'm unsure of when I'll be able to install Snow Leopard on my production machine with FCS2 on it, but if/when that happens, I'll make sure to post to this thread with the results.
I seem to remember having to do this same thing back when I upgraded from Tiger to Leopard. I did the clean install upgrade, and I had to copy the MPEG2 component back to the proper location, as it was removed on the install of Leopard. I've read that Snow Leopard's upgrade is essentially the clean install option that was on older OS installs.
So, based on that, here's my theory: Since my QuickTimeMPEG2.component was installed in /System/Library/QuickTime, it got overwritten by the default directories and files located on the Snow Leopard installer. Since the MPEG2 component isn't included on the Snow Leopard install, it wasn't included. I'd assume that if any other codec was installed in that location, it would be removed as well. But, since most codecs are installed in /Library/QuickTime/ or ~/Library/QuickTime/ codec loss on a system upgrade usually isn't a problem.
So, if I go to work on Monday, and see that my production machine has the QuickTimeMPEG2.component installed in the /System/Library/QuickTime/ location, I can only assume that it'd be deleted if I install Snow Leopard. However, if the codec is installed somewhere else, like the /Library/QuickTime/ location, it'll probably still be there when I install Snow Leopard, and there won't be any issues. ]]]
If this is true (again I have not done this myself), then copying the proper components back into the correct folders of Quicktime might be able to fix this.
Please let me know if this helps anyone.