Edit: This question probably doesn't make much sense, in light of the consensus of answers over in this question.
My best understanding (which isn't great) is that most search engines will treat subdomains as separate from one another and from the raw domain. So links to pages on
aaa.example.com do not help the page rank of pages on
bbb.example.com or on
example.com. That makes sense for (say) blogspot, where the subdomains are for completely different, unrelated blogs.
Is there a way to tell the engines that the subdomains are related and should be lumped together? Some
meta tag, for instance, telling the engine that pages from
bbb.example.com really are part of
example.com? (That specific mechanism would be open to abuse and so I assume it hasn't been implemented or at least not without there being more to it — it's just an illustration.)