This question came up a lot during the primaries, when it wasn't clear yet if Hillary would give up. The rules in question should apply to both parties.

Most states don't seem to have any laws forcing a delegate to vote for who they were supposed to vote for. So technically, there's nothing legally stopping most of them from switching their vote.

The parties of each state might have rules against switching, but it's not entirely clear.

It's actually a moot point, because only people loyal to the candidate are chosen to be delegates. Romney chooses all his delegates, McCain chooses all his delegates (well, their representatives in those states do the choosing). Then people vote for which set of delegates they want, though it is made to look like they're actually voting for the candidate.

So there's virtually no chance that any of the delegates would want to change their minds. Chances are astronomically lower than enough of them would to make any significant difference.

So basically, McCain/Palin is the ticket and there's no real chance of that changing unless they drop out.