Bill Gasarch has an interesting post up comparing attendance numbers at SoCG and CCC. He introduces the interesting notion of 'edge people': people in related areas who wouldn't normally attend the conference, but would because of locality.
This of course makes me a little worried about 2010. We have a rather large 'edge' community at the U. of Utah itself within SCI. But Utah as such is fairly geographically isolated (and indeed, there's a general belief that our department suffers a little in perception as a consequence), so we don't get the 'edge' crowd that places like NYC and Maryland potentially get.
I say 'potentially': as always, it's probably possible to get data on this. Specifically, if one had access to historical attendance lists (not numbers) for SoCG, one could first identify candidate 'edge' people (either by subjective eyeballing, or even more numerically as people who only show up "rarely" and publish "rarely"), and then figure if there's a correlation between location and presence of an edge crowd.
Similarly, one might even do this for CCC vs SoCG although as one commenter points out, the larger number of total unique authors on SoCG papers might be sufficient to explain the discrepancy in attendance (Paul Beame's argument about competing conferences is also valid: I couldn't imagine going to FOCS, STOC, SODA and SoCG every single year, in addition to the various DB/data mining conferences I attend).