I continued testing and found that the crashes continue, even when trying to capture DV footage from a MiniDV cam. Downconverted 30P results in the same crash. Same blank capture interface and same color wheel of death result when I click Capture Now.
I trashed my preferences. No help.
Also, I have sucessfully captured, on this FCP 4.0 system, XL-H1 downconverted footage (30P) prior to this series of crashes.