Originally Posted by Airburst

Strange... I did encounter a Shimano cartridge BB that was actually a cup-and-cone unit, with the cones threaded onto the spindle and held in adjustment with a locknut (I suppose it was still a cartridge in that it was removeable as a unit). One of those could potentially work loose in a fairly short time if the locknut wasn't tight enough, but the fact that you don't have play with the cartridge out would tend to rule out the bearing.