In the final stage of my iPhoto 8 to Aperture 3.23 migration I imported my a 62 GB iPhoto Library of about 18,629 "Photos", of which, by keyword, 259 were actually "Movie".
Yes - iPhotos support of Movies is half-hearted.
In iPhoto all but one of the 259 videos displayed with a preview image.
After the 8 hours of importing and processing were done I had a 66GB Aperture Library. The first thing I did was check the Movies.
This time there were 270; 259 with the keyword iPhoto Original and 11 with keyword iPhoto Edited. Aperture stacks them as it does photos (option-;).
Of the 270, 36 showed icons indicating they could not be played within Aperture -- though they could play in QuickTime. Another 43 could play in Aperture, but they had no thumbnail (black thumbnail). There didn't seem to be anything different about these movies. Another two had incomplete thumbnails.
There's nothing to be done about the unsupported video formats -- they area good reminder that the 2010 video format situation is a bloodymess. I will put these on my list of videos to transcode to a modern format -- hopefully without too much quality loss. 
The missing thumbnails seemed likewise intractable. I couldn't find any web resources on fixing them. Aperture Help was more ... helpful. Turns out there's now a menu command to generate thumbnails (new in 3.x), that took care of the problem.
Alas, there's no fix for a much more severe problem. Aperture imports iPhoto image Titles and Descriptions, but not iPhoto Movie Titles and Descriptions. The Aperture "Captions" for these movies were empty.
 The more I learn about transcoding video, the more terrible it seems. To do it well seems to be dark magic involving several pre-compression processing steps. Maybe I'll transcode to Motion-JPEG and be done with it.
Update 4/2/2012: The Title and Captions for the movies appears to be tied to the "Version Name" bug that can affect importing of iPhoto Titles from images. When I set Version Master = Master Filename on iPhoto import I got Aperture.VersionName and Aperture.Caption values from iPhoto. I also noticed that the Built in "Videos" smart album doesn't show videos Aperture can't manage internally -- so it's misleading (another bug).
The unsupported video format may be from our Canon SD camera, and there are some odd bugs there too. I can open the unrecognized video in QuickTime, save it as .MOV (same data, only the metadata changes) and import it back in -- and now Aperture will recognize and play it. So the problem isn't a codec issues, it's packaging/metadata problem. During the export/import process The Version Name was lost (not to surprising), but in addition the prior version name was set to the Date Created (bizarre beyond words).
Update 4/3/2012: I'm still trying to understand what Aperture is doing with movies imported via browser vs via iPhoto Library import. I think both methods are unforgivably unreliable and buggy, but they have different bugs. I'm experimenting with combining both, then reviewing metadata in list view to decide which to keep.
I recommend using the list view and adjusting metadata to show file size, project name, file name, version name, caption, rating, date and so on. I discovered that several videos were tagged as "iPhoto Externally edited", but they were in reality JPEG thumbnails.