Excellent [Matt Cutts link][1] from John Conde:

> For several years Google has used something called “host crowding,” which means that **Google will show up to two results from each hostname/subdomain of a domain name.** That approach works very well to show 1-2 results from a subdomain, but 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 in the future.

> This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from ibm.com. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries.

The "host crowding" has been documented before; it means that many hits from the 'same' domain may be grouped and suppressed. This extends to subdomains `x.example.com` and subfolders `example.com/x` equally, but does *not* extend to results at `example-a.com` and `example-b.com`.

And most importantly, in context of the question:

> A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example

Which is basically our goal. Well, that, and cutting the gordian knot of 25 (and counting) impossible naming situations.

[1]: http://www.mattcutts.com/blog/subdomains-and-subdirectories/