Depends on cad package and your need for renderer, rhino has its own digital renderer and so does MOST 3d cad systems. Also you can get a number of third party render descriptor converters to most packages. I know that at least solidowrks, catia and pro|e have direct mental ray connections. Ive also exported both data structures to RIB (Renderman interface Bytestream), rudimentary porting took week to do (b ut only cos is knew nothing when i did so)
Rhino last i used it spitted out directly rib files so aching up any of the free or not so free renderman spec renderers and ones that can port stuff form its description should be a breeze. Tough to be honest writing one yourself is usually no big deal. IT is after all usually pretty trivial stuff. Getting something usefull done thisway is off course another thing