CNN is full of crap as always. Now that FLA moved up their primary, NH has already moved their primary filing date to mid Oct and the rest of the early states are sure to follow.
The field of candidates is set now, because there isn't enough time left to file everything for the early state primaries for anyone without a campaign in full swing.
Florida is being manipulated to push forward their vote to make IA NH SC and NV push theirs forward. I think it’s on purpose to make it harder for someone ...like Palin ...to come in late. I think Romney is sure he has this locked up if he can hold off Palin.
The filing requirements vary. FL, for example, requires a list be submitted, by the PARTY, not by the candidates. That's it, a list from the party. Seeing as how Palin was VP nominee in 2008, I suspect she can get her name on that list with short notice.
I've checked IL requirements too (need at least 3,000, no more than 5,000 IL signatures on petitions), but not any of the other states.
Participation in (and winning) the primaries provides a commitment of a certain number of delegates at at least the first round of voting during the party's nominating convention. I believe a late entrant (skip primaries before Super Tuesday) can perform the role of "spoiler."