I think a huge factor is your motivation. Post-bacc programs want students who will be accepted to med school. A 3.1 does not show a student who is both capable and serious about entering med school. Why will you do better in a post-bacc than you did in college? Did you have some momentous experience that gave you focus and direction that you previously lacked? Is your GPA just due to a horrible freshman year or are you still hovering around a 3.1 in your senior year?
Your GPA is definitely at the low end, but it is certainly possible to get it to both a post-bacc and med school (I did with a 2.9 undergrad). However, I took time off after undergrad and worked in clinical research for a few years. It was only after that experience that I decided to enter medicine. I had excellent letters of rec from the doctors I worked with and high SATs. Even so, I had to prove that I could handle the courses by taking G-chem as a non-enrolled student before I was admitted.
If you have a good explanation, then go for it. Otherwise I would strongly suggest taking some time off to prove to both the post-bacc programs and med schools that you are serious about medicine. Even with a strong postbacc GPA (3.85) I was asked about my undergrad GPA at every interview. Admission committees are willing to forgive if you show growth, but they don't just take your word for it. They are science people...they want evidence.
Oh, and I went to USC and definitely recommend it.