Thanks to James Bennett, I learnt how to make ringtones out of my MP3s.
So here is the Dr Horrible Main Title and the Bad Horse Phone Call as .m4r files which you load in to iTunes and sync with your iPhone.
(you may need to right click to download them)
Other songs I've done are available as MP3s from my post
but they are too long to make into ringtones (they won't sync with the iPhone) so I have to pick which 30 second extract in each case to use. That page also has versions without vocal lines for use in Dr Horrible karaoke!
Enjoy! And go buy Dr Horrible from iTunes while you're at it :-)
UPDATE: Made the link to my existing MP3s clearer as at least one commenter missed it.
The original post had 25 comments I'm in the process of migrating over.