Wiennat

All mysteries has been solved.

It's pronounced like "jif". Period. The end. That's final. End of story.

Still not convinced? Read more at The GIF Pronunciation Page.

via Simon Willison's blog (Don't forget to read the comment)

Updated Jun 17 : AT&T Labs Text-To-Speech properly pronounces "GIF" as "JIF". You can download and listen to the synthesized speech here.