Mitt Romney "entering" the race for the nomination isn't exactly news; he's been running for president for years now, and running for 2012 since about the time that the last votes were counted in November 2008. Or earlier. His change of status is important to accountants and lawyers, but it's not important politically.
That said, I've been using these announcements as an excuse to say something, so I might as well continue. Romney's a fascinating candidate, because there are so many reasons why he shouldn't get the nomination (in increasing order of importance, in my judgement: health care, religion, abortion), but yet here we are. He's obviously a serious candidate with a real chance of winning.
Moreover, since I said basically the same thing about Tim Pawlenty's chances about three weeks ago, little has changed. There are still only three candidates actively running with a realistic chance of winning (those two, and Haley Barbour). At that point, I identified five others (Huck, Daniels, Perry, Jeb Bush, and Palin) who could plausibly still get the nomination. But every week that goes by without any real action on their part makes it just a little harder.
I suppose I could add that every week that goes by also makes it even more unlikely that some other dark horse will emerge. On the margins, it's probably also true that every week without one of the other five moving close to active candidacies makes it somewhat more likely that one of the implausible current candidates (Newt, Bachmann, Santorum, etc.) winds up winning, but again, we're talking about very small chances here.
At least, that's how I see it. Intrade doesn't agree. When I first said there were eight plausible candidates remaining, Intrade gave those eight a combined 72% chance of their market. That's down to about 65% today. So I continue to think that collectively my group of eight is seriously undervalued.