Rush allowed Huckabee on his show at Huckabee's own request, supposedly to set the record straight about why he was opposing Mitt Romney as vice president (not because of his religion, but because of his views according to Huckabee). He, of course, used the occasion to reiterate to Rush's vast audience why, in his view, Romney wouldn't be a good choice for veep. That kind of gamesmanship, of course, is what makes Huckabee a formidable politician. Doesn't convince me, however, that he's a particularly good (or given the statement above, a truthful) guy.
All the coverage of the Clinton-Obama drama at the Democrat Convention emphasizes how profoundly destructive intra-party infighting is. For Huckabee to continue to carry a grudge against Romney because Romney criticized him in the primaries (as Rush pointed out, that's what competitors do in primaries) is unfortunate and destructive -- and, if nothing else, highlights why he would be a profoundly unsuitable choice for any high office.
Update: Back in February, I noted Huckabee's penchant for placing personal political advantage above princple or party. He's demonstrated it once again today.