hey dexadental. well, like you i was/am planning on doing it for financial reasons. after reading a bunch of posts on here, i see that people highly discourage this, saying that unless you have a strong desire to be a soldier first and a dentist second, you shouldn't do it. a lot of people who are actually in the program had a lot to say about it, not just those speculating about it. some like it, but some say they "wouldn't touch it with a 10 foot pole" if they could do it over again. it's hard to know if you, or i, or anyone else would like it based on these few comments. from what i understand, you are at the mercy of the military, and don't have much say in where you are stationed or what you do. people also had a lot to say about how unorganized it was and that there was a lack of respect for the healthcare providers by the soldiers. again, this is all based on comments i've read on this site (MANY comments might i add), not my personal opinion as i have no experience with this.
as for the active/inactive duty/reserve time. basically, once you sign the dotted line to accept the scholarship, you are officially THEIRS. your inactive duty begins at that moment. so, technically, it is 4 years of active duty once you graduate, and 4 years inactive. if we got the scholarship for all 4 years of dental school, those count as our inactive years. this means that they could potentially call us to fight while we were in school if they were desperate but this rarely, rarely happens (although it has).
i'm not sure how much research you've done on this, but here is a site i've found very helpful. it has a lot of information, but it's worth it to read it through. especially pay attention to FAQ #6, where he addresses doing it for monetary reasons.
http://lukeballard.tripod.com/HPSP.html
i'm not trying to discourage you. i myself will probably still apply, and make my decision whether i'll pursure it AFTER i see if they'll take me.
also, there is a forum called "military medicine" under graduate forums that you can check out if you haven't already.
good luck!