The final word, I hope, on the impossibility of a late entrant into the GOP nomination battle at this point goes to Josh Putnam, who has a data-filled post detailing all the filing deadlines and other technical problems with a new candidacy at this point.
The only think I want to add to this, other than you should definitely use his info and not the stuff that I've been citing in previous posts, is that there's a danger here of getting too caught up in the technicalities of all of this. I know -- I've been contributing to this a bit. And it's worth knowing the technical stuff, to be sure.
But step back, and you'll realize that it's been too late for months. When I was talking about this in 2010 and 2011, I speculated that the real window closed for most candidates in spring 2011, and for the most well-positioned candidate on the sideline -- presumably Jeb Bush -- sometime in the summer or late summer. Oh, a candidate certainly could have technically entered the race in September 2011. But a true new entrant at that point would be starting with an enormous disadvantage in resources, whether we're talking money, or organization, or candidate experience on the campaign trail. We saw some of that with Rick Perry, who wasn't really a complete new entrant (remember, he had already published a campaign book before he got in, and certainly had been prepping for the race for some time). It would have been that much worse for a real new entrant.
One of the reasons Mitt Romney has done so well this cycle is precisely because he's been running for president nonstop for about seven years. You don't have to do it that way; Barack Obama really did enter fairly late in the process, as far as I know. It sure helps, though.