Sing a song using the lyrics to another song?....he's done it several times live.
The latest example is on the new 21 nights cd, where he sings to the tune of 3121, but sings the lyrics of DMSR. I've never personally heard another artist doing it as consistently and well as Prince does.

I like how Prince does that on "21 Nights."

It's very easy to do. Probably 90% of pop songs are in 4/4 time, so it's really not that hard - especially with vamp based songs like "DMSR" and "3121". Anyone with a bit of knowledge of time signatures/beats can do it no problems. Prince just likes throwing things in for the fun of it. He's certainly not the only one who does it. Rappers do it all the time!

No, he's not.

Prince as been doing this for ages. I remember him doing Forever In My Life (1987) and Thieves In The Temple (1992), where he incorporated It in the extended parts of the songs.

I think the best was at the 2002 Paris soundcheck, where he sang Sweet baby over the instrumetnation of Xenophobia. Very hawt.

heard him sing purple rain to jimi hendrix once

it's really not hard to do
try it yourself

I never heard him doing that, sounds great