Apple Thoughts: Convert Your Video With iTunes

"Even though the iPad has that HD feel, there are plenty of HD files it can't stomach -- the maximum resolution for videos to sync via iTunes is 720p, and anything higher (1080i or 1080p) simply won't transfer. On the iPhone and iPod touch, the upper limit is even tighter; those devices can only handle 640x480 videos, meaning that a 720p file will stall out."

If you have simple needs and don't have or don't want to use a video converter program such as Handbrake or iFunia, you can simply use iTunes under the Advanced tab to convert your files to something you can view on your iPad or iPhone/iPod Touch. Simple solution although it does take longer to covert the files according to the article.