We use a 1and1 webfarm and they just recently added Google site maps. I can asign a map to each of my subdomains/domains. Should I use a sitemap for each subdomain? Or is the single site map on the main domain sufficient?

It depends on how you have your site setup. If it is setup like a true subdomain e.g. subdomain.domain.com you will need to do a seperate sitemap for each "subdomain".

If your site is setup like this domain.com/subdomain then one sitemap will work for the whole site and any sub folders

I hope this helps!

It is a "true" subdomain setup, thanks