In iPhoto, right click (Control click) on one of the video thumbnails. Select the item "Show File". This takes you to the Finder and the actual file. Open this with QuickTime 7 (if you have it). Choose Window>Show Movie Properties and click on Visual Settings - _what does the Video Track show as the format?_ It will be an AVI file, but we need to see what's inside that AVI container.
Alternatively, choose Window>Show Movie Inspector to check the format, if opening in QuickTime X (or QT 7).
since you also use a canon camera, does yours mount when plugged in via USB?
I will have to check this later. My wife is out all day today and has the Canon with her. I've never plugged the camera in via USB, as I've always used a card reader, as mentioned in my earlier post. But I will try this for you tonight (it's just after midday here) and report back.
In the meantime, please check the format of your videos, as I've described.
John