A slight clarification to what jds2006 says, for those who are choosing graduate programs or going on the market: It's not a matter of "discipline," but a matter of "departments."
You can be studying almost anything and make it fit into an English Department.
The key problem with "Studies" degrees is that they don't SEEM to fit easily into a department, and almost all of tenure-track hiring is done in departments.
So your "discipline" (interest area) can be a variety of things, but to get a job, you have to make what you do sound like it fits into a traditional "department."
P. S. I have a "Studies" degree myself, but my research fits into several possible traditional departments.
At many of the American Studies conferences I attend, the presenters are, as sarahenne suggests, members of more traditional disciplines who are doing what amounts to their real intellectual work around the edges of their professional obligations as English or Poli-Sci profs. The Journal of American Culture that i happen to have within reach (March 2006 edition) lists the disciplines of its writers as: English; history; library science; Liberal Studies; sociology; library science; and English. It's a special edition, dedicated to Children's Literature, so it is NOT representative of most issues. Still, it does illustrate the point the previous poster made.