The 2008 Journal Citation Reports are out, warts and all. In the sciences, the journals with the highest impact factors are the Cancer Journal for Clinicians (74.575), the New England Journal of Medicine (50.017), Annual Review of Immunology (41.059), Nature Reviews Molecular Cell Biology (35.423), and Physiological Reviews (35.000). For more systems-biology focused journals, Bioinformatics comes in with a 4.328 score, Genome Biology with 6.153, Genome Research with 10.176, and Proteomics at 4.586. Nature has an impact factor of 31.434 and Science has 28.103.
Christina's LIS Rant has a re-post discussing what aspect of the impact factor leads to a rise in the number (since, as Christina asks, "Like prices and hemlines, why do impact factors always go up?"). She finds that "it's really almost completely due to the increase in the number of references cited per article!"