Here is a simple way to download MP4 videos from Youtube to your iphone / harddisk.
You can also download the HD quality video (if available) directly using this website - http://keepvid.com/
Bookmark this webpage to your favourite.
Follow the instructions as mentioned below..
Rename the file name extension from .MP4 to .M4V
Download to your iPhone using itunes and enjoy watching videos on the move..
