Recently, I pointed to a couple posts at the Space Elevator Blog about presentations at the ISDC. Now, the SE Blog has a wrap-up post on the conference. (I should also mention that there are lots of other posts on ISDC there). RLV News has some evidence that the total numbers at the conference were higher than you'd see just counting those present at any 1 time.
I went to some of the ISDCs early in the decade, and I'd hazard a guess that there were 2 or 3 times as many people in attendance at this one. There also seemed to be a more business-like atmosphere about it, as if there might be business deals, hiring, and the like going on, which is interesting since it's the conference of the grass-roots NSS, not an industry group or something like that. These seem to be good trends, although of course more attendees would be better.