Here's what I found : "...It's possible that even if Gingrich or Paul's delegates can't vote for their candidate on the first ballot, they could do so on subsequent ballots if Gingrich and/or Paul garners the support of a plurality of delegates from at least five states during the fight on the convention floor. Under Republican National Committee rules, Gingrich or Paul would need to be formally nominated after the first ballot for this to happen, and demonstrate their support in five states when this happens...".
One other thing which hasn't been mentioned is that R.P.'s delegates can abstain from voting on the first ballot.