According to the Wall Street Journal, Apple boss Steve Jobs has received a liver transplant during his medical leave of absence and is expected to return to work at the end of June.
The WSJ claims the transplant was carried out two months ago. No sources are cited other than "a person familiar with the thinking at Apple", and there's no comment from Apple. The piece goes on to say that Jobs may initially return to Apple on a part-time basis.
Given the seriousness of the claim, along with the implications that it could have for both Jobs and Apple, I would expect some sort of comment from Apple by Monday.