I found it the first Geico caveman commercial. Check it out. Doesn't the caveman on the right look just like Val Kilmer at the very end. I'm convinced it's him. As far as I know, it's not on his IMDB list of credits, though.
I've read somewhere that the Geico cavemen are played by a guy named Ben Weber and a guy named Jeffrey Daniel Phillips. I think Phillips is the Val Kilmer looking dude, and Weber is the guy in the airport ad, but I'm not sure. Of course, there was also be a third caveman (John Lehr?). Remember this one?
And this one:
The latest one, by the way, not so funny, in my opinion:
And the one I haven't seen on TV yet is funny, but I don't like the caveman as much: