Robert Weinberg and colleagues have just published an article in Nature (see here and here) where they study the role of a type of stem cells known as mesenchymal stem cells (MSCs) in promoting metastasis.
It seems that MSCs that interact with weakly metastatic cells release a paracrine substance (CCL5) that promotes metastasis. The effect works for as long as the tumour cells have access to CCL5 so these cells revert to their normal state once the factor is out of reach.
Interestingly, Weinberg reckons that this mechanism is unlikely to be breast-specific and that it could be used to explain metastasis in many kinds of cancers. This work seems to give strength to the idea that dealing with stem cells is necessary when treating a cancer.