Candid Engineer has a post on impact factors. I find the variability of impact factors by field quite frustrating as it often neglects the field specific differences in publication style. Take, for example, the field of statistics. I have yet to find a statistician who would not be proud of an article in the Journal of the American Statistical Society (JASA). It has an impact factor of 2.3; this does not compare favorably with (for example) Epidemiology (IF=5.4) which also publishes methodological papers. But the impact factor approach understates the effort required to beuild on or develop methods -- more sophisiticated methods take a lot longer to work their way into the literature.
In any case, comparing JASA to the New England Journal of Medicine (IF=50!!) makes it clear that the field has a lot to do with the impact of a journal.
So what do you do when you are on the borders of a field?
It's a tricky problem . . .