A WebmasterWorld thread has some credible sources claiming the site:www.domain.com, site command, at Google is becoming more accurate. In the past, conducting a site command on a domain may have returned a lot more pages than what is currently accessible on the main site.
WebmasterWorld admin, Tedster, said,
I also see sites returning numbers that are reasonable now when they were always 4X or worse -- and I see this even in cases where there was no canonical fix (or issue) on the part of the site owner. I think theBear got it right. The site: operator is returning better url number estimates now. Matt Cutts said that this was in the works.
This may be directly related to some folks using Google Sitemaps within Google Webmaster Central.
Forum discussion at WebmasterWorld.