I've always looked upon celebrity endorsements as a non-event or at worst a negative. I'm not impressed with Meh anyway, and in my opinion the don is a clown. They deserve each other.
Wish Ron Paul stood a snowballs chance. He has some great ideas. But I don't think he'll have enough traction.
Trump endorsing Romney also seems like a non-event to me, though I can see how it might sway some (probably few) one way or the other. It's kind of like a Democrat getting endorsed by Alec Baldwin.
There are some "celebrity" endorsement which are significant though, IMO. How about when Colin Powell endorsed Obama? Even though the Dems were all over him when he did the WMD presentation, I think they welcomed the endorsement. And it sure riled a lot of Repubs.
I think it would be similar to Hillary Clinton endorsing Romney. Not that it's going to happen!