More interesting facts about the effects of caffeine:
It has been documented that caffeine is a rather effective inhibitor of the DNA damage repair pathway in cells. Studies showed that caffeine inhibits the protein kinases ATM (Ataxia Telangiectasia Mutated) and ATR (ATM and Rad3-Related) 1. These kinases are activated by DNA damage and one of their functions is to stop the cell cycle. This gives the cell time to repair the DNA damage before continuing along the cycle. Inhibiting ATM and ATR therefore forces the cell to continue through the cycle with damaged DNA, potentially leading to permanent DNA mutations which could lead to cancer. Therefore caffeine is indeed a carcinogen, one of the warnings on a bottle of pure caffeine which can be seen in Jizz's writeup above.
However:
Research also indicates that caffeine has anticancer properties as well. In one example, scientists have shown that treating mice with caffeine reduces the formation of skin tumors after exposure to UV rays2. This may not be due to caffeine's ability to inhibit ATM and ATR, but because it is also known to be an antioxidant. Obviously, many more tests need to be done to determine if caffeine has a therapeutic use in humans.
Therefore, it appears that caffeine is both a carcinogen and an anticancer agent. While this may seem odd, keep in mind that many, if not most, of the chemotherapy drugs that are currently used to treat cancer patients are potential carcinogens as well.
Don't start thinking that your cup of coffee will turn you into a mutant or be the miracle cancer cure, since the concentrations used in the above experiments are impossible to achieve with normal or even high consumption of caffeinated beverages.
Sources:
1) Sarkaria J.N. et al., Cancer Research, 1999
2)Lu Y.P. et al., Proceedings of the National Academy of Sciences, 2002