Hello,
i got the "an invalid sample description was found in the movie" error message too when i attempted to open the videos taken with HTC Tytn (Windows Mobile). These files are saved on a PC via ActiveSync as .mp4 files and then uploaded to my Mac (10.5.4).
So I searched for fixes on the web and i found several references to changing the extension into .3gp, which worked well for me.
Given the rather high number of videos i collected in the past, i created a folder called 3gp and with Automator i created a plug-in (folder action) that replaces any file containing the string .mp4 with .3gp.
So now every time i move a .mp4 file into that folder, it is automatically "converted" into 3gp. May be there is also some way to further refine this process, for instance by excluding mp4 files that have not been created by a Windows Mobile device, i don't know, i didn't investigate yet if i can filter out the files in the Finder by means of some special file property.
So, you just need to open automator and create a single workflow action containing the item "rename finder items", option "replace text".
Then save this workflow as plug in (folder action).
Hope this helps
Flavio