Here is a link to the only vid I think online of Shatner singing Elton John's "Rocketman". This is from a 1978 Sci-Fi/Fantasy Film awards show or something. I remember people talking about it for a long time before I ever saw it (I worked in a comic/sci-fi/fantasy shop for 7 years), and some considered it further proof that he was full of himself, while others insisted he had a sense of humor. Maybe it's a mix of the two, but it is entertaining....even if its a little like watching a train wreck in slow motion.

http://www.devilducky.com/media/21099/

(if anyone has a better quality transfer of that clip, I'd love to see it)

A link to a comparison of the Shatner version and the Stewie version:
http://www.milkandcookies.com/article/2773/

Thanks Z6PO for the "Has Been" mention. I didn't know of that album, but after having listened to various mp3 of parts of it now, I look foward to getting it. He makes me smile, but I understand the album is not a laugh fest. I can't resist the idea of Shatner and Henry Rollins doing a number together.