A couple of weeks ago, Google made a change in their algorithm. Prior to the change, a subdomain was considered more or less like a separate URL. Matt explained the reason that presented a problem was due to the fact that multiple subdomains and main domains had a tendency to dominate search listings for some queries - specifically, for example, elaborate long tail query strings.

"We did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen." - Matt Cutts

Matt described the changes as not something anyone should get too worked up over. Google will still return multiple results from a single domain where appropriate. However, you should no longer find whole pages of results totally dominated by one site and 5 or 6 of its subdomains.

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