Here's the Intrade Link.
Unfortunately, further perusal of the site shows that the bettors are giving Romney about 2 chances out of 3 to be the nominee.
“Considering that we don’t have a candidate and Obama has yet to have to defend the indefensible in a real debate with a real opponent, I’m not as pessimistic as you.”
Obama does far worse against a generic Republican than any specific individual candidate. Once the GOP nominates someone, let’s see where the Intrade line goes.