The tire width to rim width relationship seems to be shifting territory:

http://www.schwalbetires.com/tech_in...dimensions#rim

One important thing with fat tires on narrow rims is that you really have to keep the pressure down. I have 50mm tires on 19mm rims but I only put about 50 psi in them. A fat tire is basically pulling narrow rims apart and with enough pressure you can actually split the rim.