http://www.ppi.cc/ppi_players/kevin_haney2.htmHe does a good job on the voice and mannerisms, and his facial structure is similar enough that they can attach latex prosthetics and have him pass for Bush, even under pretty close inspection. (I still think that his eyes aren't quite close enough together, but of course he can't do anything about that .... his face is a bit longer than Bush's, but when he tilts his head forward (watch for this on the Leno appearances -- he generally does it at least once each time) the resemblance is much closer due to the foreshortening. (I think he knows this too, and does it on purpose.)
Personally I think it would be hilarious to put the makeup on someone like Tim Robbins -- Bridges seems to admire Bush quite a bit (he deliberately goes with the folksy/dumb impression of him, nothing that would really challenge Bush's ethics or intentions, and Bar herself has said as much, since she's actually seen the act). Bridges is a part-time evangelical preacher, and uses celebrity voices in his sermons (though I don't know whether he's ever used his full Bush outfit for that).