First Herman Cain endorsed Gingrich and now Trump. This should lock up the bat-shit crazy vote for Gingrich, which is the major portion of the Republican vote.
Trump has also been talking about running as a third party candidate again. Most likely this was to receive more publicity but perhaps he might use a loss by his candidate as reason to run.
Update: CNN is now reporting that Donald Trump will be endorsing Romney and not Gingrich per reports yesterday. I think we need to wait for the actual announcement to know for certain what he is doing.