Read his blog. Matt Cutts is a Google engineer who spends his time answering these kinds of questions. The poor bastard.
Unless SEO is your career, don't spend too much time on this stuff. Really. I've seen too many people latch onto SEO like it was eternal salvation with a side of ice cream. Then they descend into endless, almost Talmudically obscure discussions over 302 Found vs 303 See other.
Don't waste your life trying to divine the minute thoughts of God/Google. Just be good. Produce good & original content, link appropriately, make sure your site is fast, and watch your 404s. Get that stuff right and you are 90% ahead.
It's your job to do things that people, real people, will enjoy. It's Google's job to connect people with what they want. Never forget that. Never try to short-circuit the user to directly please Google.
In agree that 'being good' should be the core of your strategy but ignoring other factors is naive.
If you are really unsure about some minor SEO trick, flip a coin and instead try to shave 100 milliseconds off of your latency.
Google is not run by Dungeon Masters. It's not a trap. A search engine will only pay attention to some feature that distinguishes websites if it helps them improve the quality of results. And its in their interest to communicate the features they pay attention to.
Sub-domains are treated as a separate site in the eyes of search engines.