It won't be possible to install your own cert, no, but there are plans to provide SSL in the not-too-distant future using SNI (the Server Name Indication extension to TLS) which is sort of possible these days only because IE on WinXP has finally died, for all intents and purposes. Without SNI, you do need a dedicated IP address because your domain/server name is not transmitted in the clear, which means that your traffic would need to be decrypted in order to figure out which account has the key required to decrypt the traffic... and computers HATE a Catch-22 like that. With SNI, x10Hosting will be able to use its certificate authority on all accounts, so there will be no need to provide your own cert.