In X-Statix (2002) #16 (published January 2004), Myles says that "America has fifty-one states":
Since one of Myles's main character trait is his education (as a well-read Harvard student – well, presumably Harvard dropout at this point), it's unlikely that this is meant to be the character making a mistake.
I can't find any reference to this 51st state; the only relevant thing I can find is the "Fifty State Initiative" (post Civil War, i.e. a few years after this issue), which Wikipedia says is supposed to "provide a superhero team in every state of the United States."
The next page of the comic shows a map without any apparent extra states:
(In fact, this map is short a few states: no Alaska or Hawaii, but also apparently the Colorado-Kansas, Pennsylvania-Delaware, and Massachusetts-Rhode Island borders have been dissolved. The colors of the two halves of Michigan are separate, but somehow I doubt that the Upper Peninsula is that 51st state.)
So, what is the 51st state?