Well, if you are on an HSCP scholarship, you will do whatever you want. Your job during dental school is whatever everyone else is doing. if that is sitting on your A$$, then you are gettign paid to do that. But, if you are on the HPSP, then you will be required to spend 45 days on active duty if your school has that much time off during the summers. If you had to go to school during the summers like a lot of schools have gone to, then you would be credited for the 45 days on active duty although you would be in school.
If you have a couple months off, then you will be required to do the 45 days. If you don't do it the summer before d0school starts, then during the summer between your first and second years you will have to go to ODS (Officer Development School) which is basic training for health professionals and other officers like chaplains and JAG. Then, the subsequent summers you will probably do internships at a dental clinic for the 45 days. I have spoken with a few people who actually were allowed to do dental work before they were able to do it in school. This time will be beneficial to you so you know what military dentistry is like so you can have an idea of how you will be able to relate dental school to what you will do in the military.
So, don't plan on being able to go to Alaska to work for the summer. If for some reason you are allowed to do that, great. But, I wouldn't count on it at all!