Noam and Chris have both taken note of the Fortune article in which Grover Norquist expresses his satisfaction with the current, tax-cuts-for-the-rich-loving, incarnation of John McCain. Norquist says, "He was just voting against Bush in general. I think it was pique."
This has become a common explanation for McCain's liberal past. I don't really believe it. Certainly, when it comes to taxes, it's demonstrably false.
McCain was attacking the Bush tax cuts as an unaffordable, morally dubious sop to the rich as early as 1999. I wrote a cover story at the time about McCain shocking deviation from the central pillar of Republican orthodoxy. The supply-side position is the one orthodoxy Republicans simply are not allowed to question. That's why McCain's alienation from the GOP was inevitable. It's also why he had to reverse himself in order to become an acceptable 2008 nominee. All this is explained in my book.
In any case, the "pique" theory gets the causation backwards. It was McCain's heresy on taxes that caused the GOP establishment to turn against him, not vice versa.