The latest issue of Epidemiology features a (only somewhat tongue-in-cheek) article by Miguel A. Hernan: Epidemiologists (of All People) Should Question Journal Impact Factors. Well worth reading and thinking about:
Developing a good impact factor is a nontrivial methodologic undertaking that depends on the intended goal of the rankings. Hence, a scientific discussion about any impact factor requires that its goal is made explicit and its methodology is described in enough detail to make the calculations reproducible. Paradoxically, the methodology of the impact factor that is used to evaluate peer-review journals cannot be fully evaluated in a peer-reviewed journal. As illustrated above, a manuscript describing the Thomson Scientific impact factor would be a hard sell for most journals, and hardly acceptable for the American Journal of Epidemiology, the International Journal of Epidemiology, or Epidemiology.
The same issue also features several interesting responses: