This one is my favorite solution. Simple, sweet, worked without a hitch. As far as iPhoto vs iMovie, it doesn't really matter where your movie is, as long as iTunes can see it. Currently, my iTunes Photo import works from a folder I specified that happens to contain (you guessed it) all my photos. I don't use iPhoto; I use my own personal photo library folder. Within the folder, I keep several sub-folders, and in iTunes, those subfolders show up as options for import. I select only the subfolders that I want copied to my iOS device.
I then created a new subfolder called "Camera Roll Movies" within my main photo folder, and it became an additional option in iTunes. I checked that subfolder, and also clicked the checkbox for "Include videos" (as mccabefamily suggested). Now, I just moved the movie I want copied to my Camera Roll to that "Cameral Roll Movies" folder, synced the device, and viola! The movie is now available in my camera roll. Unbelievably easy.
If you're using your own Photo library photo as your Sync Photos option, and if you're movie was created in iMovie on the Mac, simply export the movie to your own "Camera Roll Movies" folder (if you don't have one, just create it).
If you're using iPhoto as your Sync Photos option, then you will need to do as mccabefamily suggested, and get the movie into your iPhoto collection. Not being a user of iPhoto, I'm not sure what that entails, but I imagine it is relatively easy.