I've actually wondered this myself from time to time, without ever bothering to look it up. Well, it's actually happening. That WSJ article says the following, which just blows my mind:
Researchers at the Rogosin Institute are taking tumor cells from mice, encapsulating them in beads made from a seaweed-derived sugar called agarose, and implanting them in the abdomen of cancer patients. There, cells in the beads secrete proteins researchers believe could signal a patient's cancer to stop growing, shrink or even die.
So far, at least 30 patients have been treated with the cancer beads in an initial human study, and a phase two or intermediate-stage trial has been launched—with the approval of the U.S. Food and Drug Administration—to test the technique in patients with advanced colon, pancreatic and prostate cancers.
You can find the info on the Phase II trial here; they are currently at the "recruiting participants" point.
[The treatment] consists of small beads that contain mouse cancer cells from a mouse kidney cancer cell line. The cells in the beads produce substances that have been shown to slow or stop the growth of tumors in experimental animals, veterinary patients, and 23 humans with different types of cancers in a Phase 1 safety trial.