The following method appears foolproof when done on a Mac. Haven't tried it w/ iTunes on a PC.
1. Plug in iPhone4 to Mac. Run iTunes if it does not run automatically. Select the 'Movies' icon on the left. You need to be viewing your movie library for the procedure to work.
2. Run iPhoto.
3. In iPhoto, locate the videos you want to import from your iPhone. Select each video in turn, and import each into a separate iPhoto event.
4. Open a Finder Window
5. Locate your Pictures folder and navigate into it. You should see an item called 'iPhoto Library'. Right click and select 'Show Package Contents'
6. Navigate to the 'Originals' folder, and then to the <year> folder for the year in which the movies were taken (most likely, 2010 right now, given when the iPhone 4 was released!)
7. Your movies are hiding in folders named for the events. The are .MOV files.
8. Select the movie you want to transfer, and then just drag and drop it into your iTunes Movie library.
iTunes does the rest, copying the file, indexing it and displaying it in your Movie library. You can rename the movie once it's in iTunes to something other than the default 'IMG_nnnn.MOV' format.
If you then go to the 'Movies' tab, you should be able to select the movies for sync to the iPhone (which is handy, since they then show up as iPod videos, my preference). You can then safely delete the from the camera roll (and from iPhoto, if you like).
That's it!
- Matt