1990osu, since the Civil War, the Supreme Court ruled that no state could legally secede, based rather flimsily on the line, "We the People of the United States, in Order to form a more perfect Union..." from the Preamble. At least, that's what I read somewhere. I'm in a hurry and can't back it up right now. Anyway, I still agree with the ruling because we need a strong federal government to maintain our unity. But Texas indeed can legally secede because it was worked into their constitution, something the federal government would not have allowed for any other state joining the union.
Interestingly I read in the news today that Puerto Ricans voted in a non-binding referendum for statehood. We may see a 51st state not too far down the road.