Ways to market sub-domain

First thing you need is to get ranked in google
best thing to do is to get a couple of inbound links pointing to your main site, which should be linked to your subdomains, then you want a google XML sitemap, and possibly an ROR sitemap.
This will get your subdomains ranked fast.
Also some rotating content using a PHP script should keep Google/MSN interested.

I will go dig out a couple of links for you if you would like.