At this point in the process I would try to stop worrying. I got the same e-mail (also with "Dear Applicant"), and did phone them to check I had actually gotten in. I'm definitely stressed since I don't know anything about funding yet, but at least the acceptance is in the pipeline. Btw, they did say a formal acceptance will be sent next week so no worries. I know many grad schools don't seem to have their act together, but sending out a mistaken acceptance email sounds like the worst nightmare of administrators, and I would think they have checked their email list several times before letting it out.