I don't know of any references, but I do know (from reading the net) that
someone in Oregon is using agricultural/gardening fairs to market "bee
houses."  These are not for bumble bees, but for carpenter bees, which are
closely related.  It's a wooden nest block.  Since carpenter bees are
territorial, this has limited use, though, as only one nest can be supported
in a given area.  (Interestingly, my Apache uncle used to hang up nest
blocks for carpenter bees, too.  He claimed they brought luck to your house.)
I haven't heard of anyone trying to cultivate bumblebees, but it would seem
like a logical progression---*provided you figure out how to create a suitable
nest site to attract bumblebees.*  It's easier with carpenter bees, as they
are attracted to wood blocks with appropriately-sized holes, much the way
you can lure birds to a birdhouse.
 
Carpenter bees seem to be the primary pollinators of my sweet peas, being
one of the few bees with enough body mass to depress the keel of a large
modern hybrid sweet pea.
 
  -Jane Beckman  [[log in to unmask]]