Cable Card is currently only 1-way, so anything that requires your STB to communicate with the cable company is out, such as PPVs and VOD. However there's a 2-way standard being worked on (which of course will not be in any way compatible with existing Cable Card devices).

Here in Toronto, the cable company Rogers currently has NO plans to support cable cards and, according to a friend who works there, barely even knows of their existance. A shame, since more and more products are coming with Cable Card support. (No, there's no mandated support here as in the US.)