Someday it'll be that way, when everything is standardized and vendors adhere to the standards. It doesn't make much sense for Cisco or Nortel (or 3 Com, or anyone for that matter) to allow other vendors phones to connect to their systems. There's money to be made in the parts/instruments. Just like a Ford starter won't bolt to a Chevy engine.
On the BCM 200/400, you can register a "dumb" H.323 compatible terminal to the system. I'm not sure if that is possible with SIP phones on release 4.0 or not, but the day will come when it can happen.