iMovie HD 6 won't capture from Canon ZR800 - have tried...

At my wit's end here, folks.

iMovie was capturing fine last week, now it's not. Did a software update cause the problem? Is there a way I can revert to last week's iMovie settings?

Solutions I've tried from related posts on this forum:

1. Disk and permissions repairs using Disk Utility (all okay);
2. Trying different cameras (my school has four ZR 800's, none worked);
3. The "open GarageBand, play a few notes on piano, close it, and then try iMovie again" trick (?) - didn't work;
4. Trying different firewires (tried three different ones, two different manufacturers).
5. Unplugging my MacBook (2GHz Intel, OSX 10.4.10) for a few hours, then plugging it back in.
6. Turning everything off and back on a million times.
7. Starting new projects.
8. Checking settings for movie - the DV is selected.
9. I have not done anything silly like plug the firewire in backward.
10. Both my G4 Powerbook and my MacBook do not recognize the Canon ZR800 suddenly. Weird.

The only theory I have is this: one of the cameras didn't have date-time set. I followed iMovie help instructions (remove lithium battery, replace, set date-time, restart MacBook and camera) - nada.

So I'm in need of serious help here. These are student projects at my 1:1 Apple Laptop School, and we've ground to a halt.

Thanks in advance.