At Feverbee we generally recommend against parking your community in a subdomain (ie community.yoursite.com) in favour of a subfolder (ie yoursite.com/community) or giving it a domain of it's own (as we have here) because it tends to take away from the community's sense of identity. When it came to defining the URL for our new Feverbee community post migration, I hit a bit of a snag, so I thought I'd write about it in case anyone else comes up against it in the future.
Unfortunately, subfolder hosting isn't easy (unless you host everything on your own server). In our case we'll be running a WordPress instance on a server in a server farm, and our forums will be hosted at Discourse. That means we'd either need to proxy all our traffic through Discourse, or proxy Discourse through our WprdPress server. There is a third option (using a service like Fastly) and you'll likely hear claims that subfolder hosting in this way is "slightly more complex" than subdomain hosting. Don't be fooled – it is way more complex. In Jeff Atwood's words
It's like an order of magnitude, maybe two orders, more complex to route a subfolder to a different server.
Complexity aside, you have to take into account the performance hit as well. In short, it's not feasible for us.
What about the SEO implications of a subdomain?
The guys at SEOMoz recommend against subdomains because they say that it's is potentially detrimental to SEO. It has also been discussed at meta.discourse (note: a subdomain). Jeff Atwood recently confirmed for me that Matt Cutts of Google told him in a private email that SEOMoz "can't tell correlation from causation". Given that fairly convincing point from the guy that knows more about Google than anyone else, I'm ok with the decision that we've made in this case.
So in summary, from a community/science perspective, a subfolder is preferable over a subdomain, but from a technical perspective, it's not always feasible.