Hmm I was unaware of a stage version of "Princess Bride".  I seem to remember reading somewhere that Adam Guettel ("Light in the Piazza") was doing a musical version of it for his next project.  Could be wrong though.

A quick search on findaplay.com didn't come up with any results.  It's possible that the company you saw had done the show did an original adaptation of the story, or cheated and did the movie script without permission.

Here is a web site about it:

http://www.darkroomsf.com/princessbride/

although it doesn't say anything about it being officially sanctioned by the copyright holders.

Dan