Looks like variegation. It's somthing that has to do with the genetic make up that causes a lack of chlorophyll (Kind of like vitiligo in humans where they have a lack of melanin in spots of their skin). Its rare but it can happen sometimes, there was a thread about it on here (I cant find it anymore) that this happened, your plant should still be healthy just one of natures glitches.
Found a little bit of info on it about 2/3rd the way down the page on this link
hxxp://www.cannabis.info/USA/library/2401-mutant-mary-jane