The 2007 Impact Factors are now out (yesterday, 17 June 2008). Although noted in this forum and elsewhere that Impact Factors are a flawed metric, readers of this forum might be interested in some of the new figures:
All listed journals, by Impact Factor
The highest IF is of a journal called CA-A cancer journal for clinicians, coming in at a huge 69.026. The rest of the top ten are:

(2) New England Journal of Medicine (52.589);
(3) Ann Rev Immunol (47.981);
(4) Rev Mod Phys (48.403);
(5) Nature Reviews Molec Cell Biology (31.921);
(6) Ann Rev Biochem (31.190);
(7) Cell (29.887);
(8) Physiol rev (29.600);
(9) Nature Reviews Cancer (29.190);
(10) Nature (28.751)

The 2007 Impact Factors of the Nature journals publishing original research are:

1 NATURE 28.751
2 NAT MEDICINE 26.382
3 NAT IMMUNOLOGY 26.218
4 NAT GENETICS 25.556
5 NAT BIOTECHNOLOGY 22.848
6 NAT MATERIALS 19.782
7 NAT CELL BIOLOGY 17.623 4
8 NAT NEUROSCIENCE 15.664
9 NAT METHODS 15.478
10 NAT NANOTECHNOLOGY 14.917
11 NAT PHYSICS 14.677
12 NAT CHEM BIOLOGY 13.683
13 NAT STRUCT MOL BIOLOGY 11.085
(Nature Photonics and Nature Geoscience are not old enough for Impact Factors, yet.)

You can create your own lists of journals by subject area, title, Impact Factor or publisher, at ISI Web of Knowledge.

There is a free-access account at the ThomsonISI website which explains how the Impact Factor for journals is calculated.

*Note, I have amended this post slightly, because when I wrote it I didn’t realise that the links provided to the lists of Impact Factors are not unique links. What you see when you go to ISI Web of Knowledge website is a list made by the last visitor.