According to ABC News, the Romney campaign has not, at this time, begun to vet Marco Rubio in any way, "a strong indication that he is not on Romney’s short list of potential running mates." It's certainly possible that this is merely a feint; if everyone thinks Rubio is off the table, then it'll be extra surprising and exciting when he's eventually picked.
But it's more plausible that the Romney campaign simply decided: (a) that Rubio, with an entire year and a half in the U.S. Senate under his belt (and try to name something he's accomplished so far), isn't ready to be the vice-president, or (b) that Rubio meant it a couple of months ago when he promised that he would turn down the No. 2 spot if offered it. If Rubio really is out of the running, the likelihood of Romney selecting one of his many Boring White Guy Options — including Rob Portman, Bob McDonnell, Tim Pawlenty, and Mitch Daniels — just soared. For what it's worth, our money is on a not-boring white guy: Chris Christie.
Update: Romney has refuted the report that Rubio is out of the running, saying that the Florida senator is being "thoroughly vetted":
"There was a story that originated today apparently at ABC based upon reports of supposedly outside unnamed advisers of mine. I can't imagine who such people are. But I can tell you this: they know nothing about the vice presidential selection or evaluation process. There are only two people in this country who know who are being vetted and who are not: And that's Beth Myers and myself. And I know Beth well. She doesn't talk to anybody. The story was entirely false. Marco Rubio is being thoroughly vetted as part of our process."