I believe he slipped out of character in order to emphasize that "Steve Bridges is a nice guy" -- he had the same tone in his voice, as when he defended Congressman Wexler. I suspect (though I can't prove it) that they met backstage during the dinner, and Bridges said something encouraging to him. Possibly even after the speech, which Colbert would certainly have appreciated!
Bridges has been a Bush supporter in the past, but if he is now critical enough of the administration to do gigs with Streisand (or professional enough to play along even if it means it'll attract the ire of FOX News), I'll give him points. Besides, he does work pretty hard (flying across the country dragging along a bunch of equipment, then going through several hours of makeup, and doing a one-man show, not onstage but right in there with the crowd).
He's along on the Canadian leg of Streisand's tour now -- and the Globe and Mail critic blamed any problems on the writing, more than on Bridges himself. (I should mention that up here, the CBC family hour comics, and even Canadian politicans, take some pretty fierce jabs at Bush, so anything short of that might seem a bit weak by comparison.)
"She was so good-humoured and generous with her audience, how could they not forgive the silly political comedy? Besides, Steve Bridges does do a good George W., the shtick just needs some writing."
http://www.theglobeandmail.com/servlet/story/LAC.20061017.BABS17/TPStory/?query=streisand