Hello Cheapster,
I don't think .m4a files work in iTunes. I have 70 different video files in my iTunes library, and none of them are .m4a.
Here is a list of what i DO have:
.mp4, .mpg, .mov, .m4v.....
You can use a program called "iSquint" to convert these files to the correct format.
iSquint can be downloaded from here
http://www.versiontracker.com/dyn/moreinfo/macosx/28250
When you open it, make sure "Optimized for iPod" is selected, and "Add to iTunes" is checkmarked. Once the conversion is complete, it should automatically add the file to iTunes, and you are then able to delete the original file.
Hope this helps. <Edited by Moderator>
Jason