Two very useful tools for getting video on an iPhone:

KeepVid – Lets you download iPhone compatiable (and high quality) MP4 files direct from YouTube – just wack the YouTube URL in the box and you’re done. Thanks Pete.

MPEG StreamClip – it’s not often you find a useful and free video tool – but this is one. Drag-drop, convert to MP4 and click “iTunes” for one of the iTunes presets. Pretty straight forward. Mac only.