[Note: this was posted before Mitt Romney dropped out]
If John McCain doesn’t get 51% of the delegates, then a brokered convention would occur, where the delegates could vote for whomever they wish.
In the meantime, if the truth about McCain (unfit to command) would sufficiently come out and/or the economy really tanks and/or another war breaks out and talk of a draft escalates, Ron Paul could win in a brokered convention.