I thought I would quote what another site had to say about anal orgasms, here it what it had to say:
http://www.winternet.com/~redright/aimgth.htm
"As the rectum is gently stimulated by the top, the spasmodic response increases and creates the anal orgasm. The anal orgasm is similar in many ways to the spasmodic response involved in penile orgasm, except that it can last for a much longer period of time; literally for hours if you (and the top) have the stamina!"
I've always enjoyed my *ahem* bowel movements, and I've noticed that the stimulation comes from the back of the rectum in that case, not the prostate area.
Could you go into more description though as to what you mean by "ahem?" I think I know what you are speaking of but not positive.