I had the "Let's Dance" guitar jangle as a ringtone because of that ultra bright, snappy and cool sound. I think that is one of the few Bowie riff's that has a 'ringtone' quality about it. I can see Ashes to Ashes with that kind of noticeabilty along with maybe Look Back in Anger and Little Wonder since they all can 'catch' a listener's ear from the get-go. I haven't tried finding a part from the middle of one of his songs but maybe I'll take a song apart with my ringtone maker app.
Currently I have Neu!'s Isi with it's playful, tremolo synth to remind me that some fucker's want my money.