After a search, I've read through about a dozen entries around the forums .. and ended up more confused than informed. So I thought I'd document what I'm after, and with luck, something can come of this post that a) helps me, and b) helps others who are after this specific instance of Drupal.
My aim is to create three sites, using a standard www.domain as the core site (which end up being a t-shirt and gamer shop), and having two subdomains under it, which will be pwned.domain (our community site, with news, reviews, forums, guild/clan tools, LAN booking service, etc), and admin.domain which strangly enough, will be a place for the admins on either of the two sites to be able to discuss things in privacy away from the userbase.
Each site will have its own content, but will share userlogin details across the board, so when someone creates an account on www.domain or pwned.domain, the same base questions will apply (and be used in the sites).