The funny thing here is that tcole recently purchased the perfect lens for portraits on DX, the 300mm f4. Got try it with a model standing on a small hill and even a full body shot will have a wonderfully washed background.
This is where experience trumps assumptions. If you have the space to separate your subject, Telephotos are great - they slim the subject as well.
Too many new photographers stand too far away and don't pay attention to the background until they get home. Good Bokeh is achieved by being able to see in your mind what the photo will look like before you press the button. It is just being aware of the total environment.
Larger formats help for sure, but you can achieve excellent bokeh with even a digicam if you take the time to learn the system you are using. I think too many don't actually take the time to learn the system, and then assume moving to a larger format or a different lens will solve their dilemma. They are just tools. Dilemma resides in the person choosing to do what it takes to become a craftsman or continuing to be an apprentice.