It’s currently not possible to map a domain or subdomain to a single page or group of pages on a WordPress.com blog. However, you can always create a new WordPress.com blog, move your pages there, and then map the subdomain to that new blog, while having your main domain mapped to your original blog.
Hope that makes sense, and let me know if you need further assistance.
What about pointing the subdomain to a site hosted elsewhere. From what I understand, that designation would have to happen in the CNAME of the zone file where the domain is hosted. Can I do that at wp.com?