tedster - 8:42 pm on Jan 17, 2011 (gmt 0)
In order to use Webmaster Tools submit any sitemap URL that contains a subdomain, I believe you need first need to verify that subdomain.
From there on, I'd say it's a matter of what is easier for you to maintain. If it's a lot of URLs, individual Sitemaps are probably the best direction. And note, you will need to add a dedicated robots.txt for each one - subdomain.example.com/robots.txt - with a Sitemap URL for that subdomain.