Motorola Droid doesn't support content protected with DRM (Digital Rights Management) such as m4p video files from iTunes. Motorola Droid allows to play DRM-free .mp3, .aac, .wma, .ogg, .wav music and .3gp .mp4 video files.

If you have DRM-protected content from iTunes, as well as eMusic, Napster, Rhapsody, Nokia Music Store, Spiralfrog, Bearshare, Puretracks, MusicGiants, Amazon Video on Demand, Cinema Now, Movielink, Microsoft Zune MarketPlace,... you need to remove DRM protection before you can use audio and video files on your Motorola Droid.

See also:
DRM removal software
How to transfer music and videos onto Modorola Droid using Motorola Media Link