Thank you everyone for your responses!
@drownsoda: Thank you for the congratulations.
I had not thought about the fact that the email is still an unofficial acceptance. In the professor's email, it does say that the formal offer of admission will arrive in the coming weeks. I have no reason to suspect my offer would be revoked, but nonetheless it is better to be safe rather than sorry. I will wait till I receive my official letter of acceptance before withdrawing any other applications.
I am quite certain that the program is a good fit. I lived on the campus for a month this summer while participating in a workshop run by the professor who would be my advisor if I attend this program. I have spoken extensively with students who are in the program, and who applied to the program, and have friends who are in related programs at the same school. I know that the funding offered by this school is the best in the field, and while I was there I spoke to other professors who I plan to work with. Unfortunately the city it is in does give me pause, but I have promised my husband I will wait to hear back from my second choice school anyway since he would prefer the other city. Long story short, I am extremely familiar with the program, and feel confident about it, but will still wait to receive my official acceptance letter, just to be safe. And then I will withdraw my applications from any programs I have not yet heard back from.