The substance of McCain's claim is pretty weak: I don't recall Bush ever saying that Iraq would be a "day at the beach," and in fact casualties to date are considerably lower than what was generally expected for the ground war to Baghdad, where you generally heard figures in the 10,000 range. (Ted Kennedy predicted that we'd run through battalions a day. Gary Hart predicted worse.) It's more the duration, and the extent of the bad press, that has exceeded expectations, really, though McCain's pretty sensitive to bad press.
But this isn't a "backstab." In fact, I wouldn't be surprised if it were choreographed by Karl Rove. Democrats forget it, but Bush doesn't matter much from the perspective of 2008, and if the GOP can get mileage out of Bush-bashing, it will.
Retrospective Reagan hagiography has obscured this, but in the last couple of years of the Reagan Administration we saw the same thing. Reagan was expendable, since he couldn't be reelected, and with the country tired of the same guy, Republicans (politicians and pundits alike) distanced themselves in order to position for 1988. Bush Sr. ran in 1988, in fact, on an "I'm not like Reagan, but I'll still protect the country unlike those weakling Democrats" platform. Whoever is the GOP nominee in 2008 will do the same, and will be able to do it more obviously because -- unlike George H.W. Bush -- they won't be a sitting Vice President.
To the GOP, Bush is a wasting asset; like Reagan at the same part of his term, he's expendable. They'll use him up, and if the best way to get value out of him over the next couple of years is to bash him, then they will. That's just politics, and McCain's just ahead of the curve. Being ahead of the curve may not be smart, since McCain's biggest weak point is with the Bush base, but I think it's the strategy.
MORE: By the way, here's a roundup of lefty predictions about the war that illustrate that many antiwar people were hoping expecting that things would turn out much, much worse than they did. McCain, however, was not endorsing their views at the time.