A solution, which has been available for a while now, has just come to my attention again.Â Vringo is a service which has a native Windows Mobile application, and allows for the uploading and downloading of video ringtones, and also with a small hack allows you to play any of your own videos in .3gp format.
You need to register with Vringo.com, where you will be able to download the Windows Mobile .cab. You can then use the software to preview and download video ringtones from a large selection.
To play your own video rename your .3gp video (less than 30 seconds long) to pre_vringo and copy it to My device/Application Data/Vringo, overwriting the previous file and restart the application.
The application does seem to have a problem with the phone canvas as found on the HTC Touch HD, but should work well on most phones.
For more information see this XDA-Developers thread here.