It works, but it varies based on which domain mapping plugin you use and what SSL settings you're using.
Two SSL options:
1. Admin only protected by SSL
2. Entire site protected by SSL
Two mapping options:
1. Site and admin both mapped
2. Site mapped, admin on host domain.
If the only SSL is on the admin and the admin is using the host domain rather than the mapped domain then all you need is a cert for the host domain.
If the SSL is for the mapped domain then you'll need to get a UC Cert, which works great but it's just a bit pricier than other certs.