My 13 yr old male cat died August 15/2010 from lymphoma/bowel mass. Some of these symptoms sound very much like his. His appetite decreased significantly. He was always a picky eater and would only eat a certain type of food, but once the lymphoma diagnosis came and he got sicker, for a while he would eat nothing but tuna, and then near the end, he would not even eat that.
We knew he was ill because he started eating the kitty-litter so we took him to the vet and they did blood work and his white count was elevated. The vet tried a 10 day course of anti-biotics in case of infection, and then redid the blood work to find out the results were worse. We treated him with oral doses of chemotherapy and prednisone for a few months but the cancer won in the end.