Finalist for fellowship at prospective grad school. Does this mean I'm admitted? I was recently emailed that I am a finalist for a fellowship for a prospective grad school I've applied to (this is for math PhD), and that I need to email back to confirm that [school] is a top choice of programs for me. I have not received an admittance letter. 

Does this usually imply that I'll be admitted? I've tried asking the grad coordinator, but she was pretty vague and said something along the lines of "After assistantships/fellowship decisions are made, the admissions chair will begin the admissions process."
I am the Graduate Coordinator for a mathematics PhD program.  But even I don't know the answer to your question (except possibly if  I'm the one who contacted you...but I don't think I sent out any emails quite like that).  So I don't think that random internet strangers can give you a definitive answer.

It's clear that you haven't been admitted yet, or the Graduate Coordinator would have told you so.  As for "usually imply" -- I am not willing to go so far.  What I can say is that being informed that you are a finalist for the fellowship certainly increases the chances you'll get admitted.  But for instance this fellowship may allow the department to admit a student that they would not otherwise be able to admit.  In that situation you would still think that they would admit someone that they put up for a prestigious fellowship ahead of most other students, but there could be exceptions.

As my favorite Harvard Square chess hustler used to say, "Sometimes you never know."