Sorry to hear of you troubles russolivant.

@singleminded: Assuming the nut was tightened correctly in the first place it would not come undone because it is locked by the tab washer.
The only way for the nut to become loose is for something to be wearing due to friction. In my unfortunate experience ( http://www.xt660.com/showthread.php?t=18016) it would appear to be the plate washer between the main bearing and the drive gear.

It also appears to me that the clearance between the main bearing and crank on the primary drive side allows the slipping of the crank in the inner race. Is this what then causes wear on the plate washer/face of inner race? and hence a loose nut which causes the crank to slip even more in the inner race which causes more wear on the plate washer.....................

If you're lucky russolivent you may get away (for awhile at least) with just fitting a new bearing.

BTW, and signs of brown slime/corrosion on the primary drive side of the crank/gear assembly?

It would be useful for "statistics" if you (or your dealer) could let Yamaha know of your problem (including chassis & engine numbers). I think they got bored of hearing from me.

Best of luck and please keep us posted...........