Yes, definitely! It's happened to me several times.
It helps to have a partner with a long enough penis to reach your prostate...but if you think about it...your doctor can reach it with his finger, so how long does it have to be, right?
All you really have to do is relax, get into it and work your pelvis to help your partner's penis rub against your prostate.
I find it helps if your partner is thick enough to make you wonder what's happening in the first place because the in and out friction on the sphincter plays a key role in this. Then once you start working it to make him feel good in return it all falls into place.
You can't just lay there and grimace cause it hurts and expect to cum, know what I mean? ;-)
By the way...I'd like to differentiate the above from "expressing" cum out of the prostate by massaging it with either the finger, hand, dildo or other device as some have mentioned before.
I have an Aneros, the Progasm model no less, and it's very easy to put that puppy in, squeeze the PC muscles a few times to rub the tip against my prostate and I'm flowing in no time, hard or flaccid. The motion against the prostate is basically a massage that expresses the fluids from the prostate. (Feels great, by the way.)
This is different than cumming, in my opinion. Cumming hands free as I described it above is really more of an anal orgasm. It has all the build-up, tension, anticipation and massive release that's usually associated with a penile stimulated orgasm...but it takes place from the inside out, not the outside in.
Come to think of it, I wouldn't be surprised to learn that my enthusiasm for dildos is directly connected to my desire to reproduce those very feelings. Hmmmm, hadn't thought of it that way before.