Hey Beagle,
I retook it because I thought I already knew the material. Big mistake!! To make matters more complicated, my pre-professional advisor strongly warned me not to retake physics, but I chose to (because I'm an egomaniac), and suffered the consequences of getting the same grade in the same course in back-to-back semesters.
Luckily, I was able to compensate that with good grades in upper-level bio and chem courses. I think the Adcoms were more impressed with those grades than the poor grade I got in physics. Also, I explained to the Adcoms, both in my ADCAS essay and in my personal interviews, my grade in physics and they were very understanding. They know that students are not always perfect and screw-ups do occur, but make sure to reasonably explain the bad grade (i.e. family death, parents divorce, etc.)to the Adcoms.
In answer to you last question, I'll be a DS-1 this upcoming August at Buffalo.
Good luck in your studies.