It is not common but neither is it very rare. I have dabbled with it on occasion. Once again I will refer to this thread about s scene which had a CNC element

http://domina.ms/~domroot/thebuzz/ubbthreads.php?ubb=showflat&Number=3510#Post3510

However I would hazard a guess that for every time it actually happens in a session there is another instance where the Domme talks the sub out of it. The problem is it can be very hot to fantasize about but the actual experience can prove traumatic. It falls under the "be careful what you wish for" cliche. It is completely inappropriate for a first session and at best borderline for a second.

Whether or not rape play is BDSM is open to debate