I was clued into these radios awhile back: http://www.universal-radio.com/catalog/fm_txvrs/900a.html
It's a very neat idea. Get a common control point and then buy swappable modules that cover the band you want. I know other radios did the same sort of thing internally: Kenwood TM 741/742/941/942, Yeasu 726R/736R, and a few others.
Kenwood even had a remote head RC-20 that would work with a bunch of their otherwise standalone mobiles. I think it worked from the X21 to the X41 series. IE TM-221, TM-241, etc. With the optional IF-20 accessory you could even hook up 4 different radios to it. You could attach radios for 2m/220/440/1.2 to it.
Neat concept, I'm sure it got expensive though. These systems were probably all phased out for single radios with everything in them. Makes me wish someone had settled on an open protocol for connecting external controls to these radios. Even make it industry wide, not just Ham Radio. Would it be against the rules to have a single control head and microphone that can switch between controlling 1 or more Ham Radios, a CB radio, GMRS, scanner, etc. Each radio could be type-certified for their task but controlled from one vantage point? I guess you would have to be careful to conduct yourself appropriate for the service you are using.