Some time ago I ran a site that had dynamically created subdomains which I found would each needs it's own sitemap as a sitemap had to be for it's own domain/subdomain. I had long forgotten about this until recently as I am creating a site again that incorporates this. It is my understanding that Google now treat a sub folder as more of a folder rather than a subdomain. Does this mean that Google sitemaps now allow url's for different subdomains or does each subdomain require it's own still? I have tried a little test on a domain I run which has a subdomain. Google has successfully downloaded the subdomain but as of yet not reported any errors within it. However did Google report errors where a url was contained within the sitemap that did not belong to the domain?
as far as i know, a domain is a separate site as far as google are concerned. so you have to generate a separate sitemap for every subdomain, and ping google separately. this was an issue for us, with our current project (see sig). every user gets a subdomain, so it can't be folded into the sitemap for the site in general - the www.domain.com site doesn't have authority over user.domain.com. but the number of pages under user's subdomains is low, and the number of subdomains is huge, and you have to create a separate "site" account at GWT for every subdomain, which is impossible. so we are not doing sitemaps for subdomains. from experience with other projects, the lack of a sitemap doesn't affect how subdomains are indexed, probably because the subdomain is the front page and front pages are always indexed.
subdomains are COMPLETELY new domains for search engines. you will need to create one sitemap for your every domains and your main domain www . sitename.com must not contain any urls from your subdomains because www is also a subdomain on sitename.com like forums or chat or images