I am pretty sure that this is because Google sees different subdomains under the same domain (ie. a.example.com, b.example.com) as different websites, and so if you got a link to a.example.com, maybe a.example.com's pagerank would improve, but example.com's pagerank will not improve.
This is why choosing one main domain for you website (either www.example.com or example.com) and 301 redirecting all requests from the one not chosen to the chosen one is recommended.
This is true. This is why we decided to set it as http://blog.dawdle.com instead of http://www.dawdle.com/blog . We have some freelancers that want to write about gaming in general (so they might get a callup to get paid), so it's editorially independent, and the blog can pass independent PageRank to the main http://www.dawdle.com/ domain.
The point is: don't automatically just shove everything in subdirectories; think about if you want your subdomains to have independent PR or not - it might make sense in certain situations.
Like I said, I'm interested in data, not speculation. Reading this is like reading a review of a power cable that makes your speakers sound better. People can convince themselves of anything they want to.
"Use directories instead of subdomains."