A subdomain is considered in the same way as a deeplink in the guidelines. It's all subsections of the overall site content and the precise structure of the URL is unimportant.
If the deeplink or subdomain contains content on a different theme from the root URL then it might be considered for listing in a different category. Compare
www.bbc.co.uk in Arts/Television/Networks and news.bbc.co.uk in News. However, listed deeplinks (and subdomains) are the exception rather than the rule.
Deeplinks and subdomains are relatively more common in informational categories, pretty uncommon in commercial categories.
Hope that helps. <img src="/images/icons/smile.gif" alt="" />