hmm....personally, I've never really doubted that Luffy and Ace are brothers. They say they're brothers, and that's all I really need for proof.
There could be many reasons for them having different family names~ and many interesting ideas have already been posted here on that ^^ The most likely to me is that Dragon is not Ace's real father, but since Garp says he trained both of the boys, I think it's equally possible that Ace just decided to change it. ANBU4U brings up a good point about him wanting to change it due to his famous, and depending on your perspective, infamous, relatives.
But even if they're half-brothers or not even related by blood at all (which I doubt, because to me they look very similar), as long as they consider each other to be brothers, then they are, imho. So in the end, having different family names or the degree to which they are related by blood doesn't really mean a whole lot to me, I suppose ^^
on a side note, to the best of my knowledge Portgas does not spell Monkey backwards, so I think this theory can be put to rest