Tevren points me too this article in the New Scientist of a new cancer therapy being developed using adenovirus (for those interested here and here are the abstracts from the lab doing the work).
The mechanism by which the adenovirus works is by necrotizing cancer cells and propogating only inside them. To acheive this, they deleted a gene called E1B-19kD that stops healthy cells from undergoing apoptosis and has no effect on Cancer cells (since they are immortal). The adenovirus replicates itself to a point where the cell just bursts, while simply killing the healthy cell. They point out a difficulty in the virus spreading through the tumor, since connective tissue and tumor matrices can get in the way, but they don't comment on a problem that I forsee. Namely that, since tumors tend to have more blood vessels, the overproduced virus might 'leak' out to healthy cells causing a temporary problem.
Oh, well, this is the brave new world of science where cancer is fought with virus', and bacteria are used for bio-defense. Just don't let the environmentalists know.