Moviefone posted an interview with Steve Carell a few days ago. It's mostly about his new flick Crazy, Stupid, Love, but right at the end they ask about his involvement with Frank or Francis:

And you're going to be working with Charlie Kaufman on 'Frank or Francis'? You two worked together on 'The Dana Carvey Show.'
Yes! Potentially. That's not a done deal but I'm hoping to. So, I would and will jump at the chance to work with him. I hope that all comes together. It's a really great script, so I'm excited about that possibility. (Source)

I really like Carell - aside from being talented, he strikes me as smart, and a nice guy. If he's in it, I kinda hope he plays Frank - the film director. It'll be confusing for me if Cage plays Frank, given that Cage has already played Charlie, and Frank is kind of another Kaufman doppelganger, y'know? It'll be like Adaptation 2. Which it sort of is anyway. But not really.

That clears that up, then.

(OR IS JACK BLACK FRANK? Surely Black is Francis, the blogger.)