There's loads of us lefties playing righties - the late Gary Moore as well as Mark King, I believe, are/were both left handed.
Someone hit the nail on the head above -you generally do what feels right unless someone tells you otherwise.
For me it was Mike Reid of Saturday Superstore fame ( not the"Pat" MIke Reed ) - I would be playing air badminton raquet, pretending it was a guitar, left handed style and suddenly I see this guy play guitar on the telly and I am now aware that I'm playing it the "wrong" way round so switch to right handed and its been that way since 1987
Oh aye - I've seen Brad Whitford from Aerosmith play a lefty strat right handed, although its probably some custom made thingy