Thinking as I'm typing (meaning I haven't formed an opinion yet haha).. but..
Well, viruses can only inject their information into cells with a specific protein on the outside, which is how the virus recognizes the cell. For this to work, cancer cells would have to be distinguishable from other cells to viruses based on their exterior. Otherwise, you're inserting something into your body capable of mass destruction (because you can't direct the virus). Since cancer cells are just cells misreplicating during cell division, I somehow doubt this would lead to a change in their protein (or sugar?) layer. Chances are, the virus would end up targetting all, say lung cells, and the consequences may be much worse than having cancer. ..If not the same, because perfectly healthy cells are still being compromised.
Man, I had an awesome idea for the cure for cancer, but I can't remember it anymore. Something about lysosomes, I think.