I am against speculating about somebody's health even if it is Steve Jobs. But today Steve Jobs himself has issued an update on his health status in a letter on the Apple site (via WSJ).
Steve Jobs says in the letter to the Apple community that the reason for loosing weight in 2008 is a hormone imbalance that has been reducing proteins. The remedy for this nutritional problem is relatively simple and straightforward according to Jobs. Steve and his doctors predict that he will have normal weight again by Spring.
Mr. Jobs also makes a point that he will be very open and straight forward with the Apple board and shareholders if he cannot do his job anymore.

So stop writing rumors about Steve Jobs' health - it is his very private matter. Glad to hear it is nothing serious. Enjoy Macworld 2009.