This is one of the most frequently asked for apps I’ve seen on the ideas stream:

Is there a app for the iphone that updates Twiter and Facebook simultaneously?

I’ve also seen it asked for the Blackberry, and for its integration with MySpace too.

I’ve got no idea if such a thing exists, but if it does, they should be charging money for it (and promoting it more) as there seem to be lots of people looking for it.

This idea was pulled from the TwitEye Idea Stream.