run a google search for 'usuhs obligation' - the first hit has a nice summary/side-by-side comparison of USUHS and HPSP, including service obligations.
to answer your question, yes, you have to serve, and the commitment is at least year-for-year, i.e., one year "payback" for every year of scholarship received. that commitment varies with service (Navy has obligatory General Medical Officer service) and with the extent to which you get financial help (if receiving officer pay as a resident you have to payback years for that, too).
i'm interviewing at USUHS and applying to HPSP for both Army and Navy. if you're really interested, you can message me to find out more, but perhaps more useful to you, check out the Military Medicine forum on this website, and start talking to military docs!