Thanks very much for those suggestions. I have done a little experimenting with them.
First, let me mention that this Canon S90 shoots movies in .MOV format. In the specs, it says "Movies: .MOV (Image data H. 264; Audio linear PCM (mono)."
On the other hand, I also have a Canon camcorder, a Vixia HF 200, that shoots movies in "AVCHD Video compression MPEG-4 AVC/H.264; Audio compression Dolby Digital 2 channel." I have no trouble importing movies from the HF200 into iMovie.
Maybe the .MOV format of the S90 explains everything?
Back to the S90 experiments. When I connect it, I get no disk image on my desktop or in Finder, as I would expect. It opens iPhoto, though, and shows up in the left column as "Canon Powershot S90". (I can, as you suggested, change it in Image Capture to open iMovie instead and it will but iMovie will still not see or import movies from it.)
I can import both still and movies from the S90 into iPhoto. With the movies, a double click brings them up in the Quicktime Player window. They play nicely.
If I then open iMovie and let it generate thumbnails from iPhoto (which I normally skip) then the movie from the S90 shows up as one of the iPhoto videos. So, yes, that would be one way to do it.
The other way, I have found, is to drag (copy) the movie out of iPhoto onto my desktop and then I can easily import it into iMovie from the desktop. I can then delete the copies in iPhoto and on my desktop, keeping only the one in iMovie. So, iMovie can see it on the desktop and import it from there but it can't even see it in the camera.
Comments?