A few weeks ago I had the same discussion with the salesforce support team. I tried to associate a domain and custom (CA) certificate.
This seems to be a documented feature:
To associate certificates with a domain: Contact salesforce.com - Salesforce docu - domains
Therefore I created a domain and contacted salesforce. But they told me that there is no way to associate domains and certificates in Salesforce.com. In my business case I implemented shop system based on force.com sites. SSL was a key feature for me.
But anyway long story short: They told me that the activision case was a very limited pilot. (And from my point of view it was limited to activition.. ;-) )
In the end the support told me that this feature would be or could be part of the roadmap and I should wait...
But from my point of view there is a workaround:
Cloudflare allows you (paid version) to implement "SSL Proxies". Using this feature you can use the custom or branded domain e.g. yourCompany.secure.force.com and in the background you can use cloudflare to send all the traffic via ssl to cloudflare and they will forward it for you to salesforce.
Cloudflare wrote an article about this topic some years ago (in particular they are talking about google app engine but basically it is the same issue). Blog Article