Yes, you can get SSL certificates that cover multiple domains -- they're usually called "multi-domain" or "universal communication" certificates, and they will have a single CN and a lengthy list of subjectAltNames. Most providers will sell you one, but it might cost you stupid amounts of money. We don't do provider recommendations, though (because who is who in the zoo changes so quickly). They're popular for people who want to do SSL on a number of domains, but don't want to (or can't) chew a lot of IP addresses (EC2 hosting, for instance).
There are 3rd party companies issuing SSL certificates which can be used for multiple domains and subdomains. These are called SAN (Subject Alternate Names) certificates. These are priced bit higher than standard 1 domain based SSL certificates. You can buy those certificates for certain number of SANs (Subject Alternate Names or domains and sub-domains). Initially you can specify all the domains/sub-domains for which you need the certificate for. But if you dont know all the domains/sub-domains right in the beginning, you can get it for few domains and then during its validity period you can get it reissued by adding more domains as Subject alternate names.