Well, the more I thought about it on Saturday night, the more skeptical I became for three reasons, the first being that I never interviewed. Another reason is that there was nothing in the envelope beside the letter (no form to mail in with my matriculation fee, etc). The other reason is that I realized the letter mentioned that my acceptance was contingent upon successful completion of my undergraduate degree. Having graduated a few years ago, I knew this did not apply to me. So, after the initial shock of the letter wore off, I had a feeling there had been a mistake, so it came as no surprise when I called them today.
However, I do feel bad for anyone who may have declined other offers of admission due to this mistake. I had the letter for 36 hours before I could contact anyone in admissions, so had I believed it, I very well could have responded to my other schools.