Dropping in just to once again second how awesome this would be, if it's done right. Yahtzee seems to be on good terms with Valve; who better to develop it? The game would probably have a campy feel like the old Batman TV show (complete with Adam West's voice as the hero, of course), since that's what Yahtzee liked so much about Saints Row 2, and Valve proved they do that well with Team Fortress 2. And you know they're not going to rush through it and turn out a second-rate game.
They need an excuse to replace the Source engine's stage-hopping with GTA-style progressive-load anyway.