Mikkel deMib Svendsen in a thread at Search Engine Watch Forums named Subdomain Spamming Good or Bad? writes that;
Subdomains work very well at the moment. No doubt about that. I can take aprominent, old, domain, set up a brand new subdomain, add one link from the original domans front page, throw up whatever content I want and within days have plenty of traffic. These days it sems that almost all linkpop value from the original domain is transfered - and I see this happening in both MSN and Google.
He does believe that this will not last forever and that Google and MSN will close the hole sooner than later. However, subdomains in general, will continue to be an important part of site structure. So the search engines simply can not do away with them.
Forum discussion on sub domains and spam at Search Engine Watch Forums.