Is it possible to use single SSL for multiple domains?

If those are the sites of on different sub-domains, it should not be a problem. More exatly:

The only problem is the location of private part of certificate used for validation — it should not be accessible via network, therefore it should better be stored on the same computer where your site is stored or at least sit in a local network inaccessible for Internet. By these reasons, if all of your sub-domains sit on the same computer this is not a problem at all. If they sit on the same local network totally isolated from Internet, it is also not a problem. (I used servers with two network cards one connected only to Internet, another one to a local network.)

If the certificate is issued by VerySign and validated by VerySign, it should not be an issue, too — technically. However, I'm not sure about legal and financial side of it. Perhaps you would have to pay more for more sites.

—SA