As others have suggested, Shatner would *never* agree to portraying anyone but Kirk, and I must agree. If Nimoy gets to be Spock, Shatner gets to be Kirk. They are, after all, the originals, and ST is a ground-breaking show (and this isn't the same as, say, Lou Ferrigno playing a security guard in the Hulk movie).
I have to say the mind meld idea was amazing! It would have given us a chance to see Shatner as Kirk, without it being too intrusive on the new characters.
Shit I can't believe they didn't roll with that.