http://www.mattcutts.com/blog/subdom...ubdirectories/
Matt Cutts Said,
"our policies for how subdomain.example.com vs. example.com relate to each other haven’t changed."
"Google will be less likely to show (say) 7-8 subdomains from one domain for some queries, but the point I’m trying to make is that the vast majority of sites won’t notice much difference at all in their referrals."
I'm thinking if you have twenty subdomains going after the same search term your fucked
if you have twenty subdomains going after twenty different search terms they will still be treated as different domains