Side question: what's the best company for SSL certificates where you're hosting multiple distinct domains for various clients on the same server? I've read about SAN certs, but I haven't found any documentation ...
DigiCert claims that you can add/remove alternative names on already purchased certificates right from their account (you don't even need to include them in the CSR just the primary), but I haven't tried it personally. I wonder about what authentication you, as owner of www.foo.com, have to undergo to add e.g. myapp.client.com to the alternate name.
The downside seems that the organization/country/city fields must be the same but that doesn't show unless you use EV The upside is no painful IP acquisition, CSR and renewal process.
As far I know the only thing that works reliably is to get multiple IPs and multiple (wildcard) SSL certificates. You can try to save a little money by getting startssl certificates (free) or by using SSL host headers (multiple SSL on one IP address), but it doesn't work on all browsers so you end up wasting time explaining to your customers why they get an error when they access their site.
If there is more than one site hosted on a single IP, the client sends a request for the SSL certificate. In the "old" way, the client didn't say to which domain it wants to connect (it only told that after the SSL connection was established), so the server didn't know which certificate to send.
The problem has been solved with SNI, but it isn't universally supported (yet), though we are close (namely IE on XP). With SNI the client basically sends the server to which domain it wants to open a secure connection, so the server can serve the correct certificate.
