I think only Terry Funk has had more heel/face changes than Big Show, only because Terry wrestled for over 40 years and wrestled for just about every promotion on the planet at one time or another. With Terry Funk, you never really knew what he was until the match started and he might very well switch sides next show. But Terry Funk is nuts and that's just who he is.
Big Show is different. He's a charming and likable guy, and a natural face. He should be a face all the time. Kinda like Andre The Giant was before he turned heel on Hogan. But the WWE seems to turn Big Show heel whenever his popularity starts to drop off a bit.
I think Show will either lose (fair and square) a big showdown to Cena, or come up short in a title match against Sheamus, and decide he would rather have the fans on his side than booing him. He might even apologize for being a bad guy to get the fans back on his side. But for right now, the WWE needs a monster heel until Orton, Barrett, or Lesnar come back. Re-masking Kane didn't work as well as they (and the IWC) had hoped.