I had one a couple of months ago on a SAN-MSP flight.
I was in an exit row on an A320 (say, row 11?) and a lady way in the back had some sort of issue... they asked for docs/nurses/EMTs etc... well.. after flying some more, the Capt came on the PA and said we were making a landing in DEN.
So we made the landing in DEN and then headed back up towards MSP.
What's interesting was that I was talking to a LAX-based off-duty FA who was going to MSP for a work meeting and he said that 'in my 17 years of flying for NW, I have never been on a plane that was diverted for a med emergency."