[tptacek]

To a first approximation, the whole US is at-will. The exceptions to at-will are expensive to litigate and have very uncertain outcomes. The "just-cause" standard that is basically the worst-case for an employer basically amounts to "there must be some valid reason to terminate the employee"; it would come in to play if you, say, fired someone for distracting you by being too attractive (actually happened!), but not if you thought you were getting poor bang-for-the-buck from a developer.

I'm not a lawyer, &c &c, but it's probably smart to deal with all employer/employee relationships where you don't have a contract addressing the issue as if it was at-will.