From what I can gather, the test for protate cancer involves looking for raised levels Prostate Specific Antigens, which are a marker for cancer. Most reports say that raised levels are also caused by having sex and cycling. There is no causal relationshop proved between cycling and prostate cancer, a subtlety to fine for headline writers to bother with.
Even the elevated PSA is disputed in some studies
http://www.bikexchange.com/sp98f2.htm
Ill keep a skeptical but open mind, but its not worth losing sleep over.