Vanessa Fox at Official Google Webmaster Central has written a post namely using the site: command. Over at this entry she has explained that a site command update is taking place to make the site more user friendly to SEOs, Webmasters and searchers.
Historically, Google has avoided showing pages that appear to be duplicate (e.g., pages with the same title and description) in search results. Our goal is to provide useful results to the searcher. However, with a site: command, searchers are likely looking for a full list of results from that site, so we are making a change to do that.
Google will very soon start showing a complete list of indexed pages.
In some cases, a site: search doesn't show a full list of results even when the pages are different, and we are resolving that issue as well. Note that this is a display issue only and doesn't in any way affect search rankings. If you see this behavior, simply click the "repeat the search with omitted results included" link to see the full list. The pages that initially don't display continue to show up for regular queries. The display issue affects only a site: search with no associated query. In addition, this display issue is unrelated to supplemental results. Any pages in supplemental results display "Supplemental Result" beside the URL.