The risk from rimming was answered by my doctor when I asked. The risk for transmission of AIDS is quite low. Yes, since the AIDS virus basically infects all cells it is present in feces and alimentary canal mucous.
However, the bigger risks involve Hepatitis, parasitic disease, and diseases associated with human waste (Dysentery, Cholera, etc.). Minimizing these risks revolves again around knowing your partner and their behaviors, cleanliness, vaccination, using dental dams, and having regular doctor checkups and tests to detect any of the associated risks.
All forms of sexual activity with the exception of j/o carries a risk of exposure to a multitude of bacteria, virus, etc. Knowing the risks and dealing with them effectively requires an open dialogue with your doctor. Also make sure your doctor stays up on latest developments and news associated with AIDS and other contagions. Some do, some do not.