At one point in time, there was an automatic qualification for med students for the Army hpsp which was a 3.5 gpa and a 25 on the mcat, but I am not sure if that is the case anymore.
One thing that I am sure about is that neither of the branches are hitting their recruitment numbers when it comes to dentists, and that includes the air force.
The proof in this is that the Air Force now offers 4yr hpsp, which has only happened in the last 2 yrs. Also, the number of "boards" that meet to select hpsp students has decreased to only 3 times a year b/c there were not enough people applying throughout the year to warrant more boards than that.
All of the branches have been registering declining numbers of applicants in medical, dental, and nursing, which is evident if you do a google search or step into a recruiters office and read the Airmen, or All Hands, or any other military periodical.
Just recently the Army lowered its DAT number to 15 to qualify for the hpsp. This is no joke, so as long as you are around a 17 or so, and/or an acceptance letter you should be fine.