sgolds, Elmer, re: single system Opteron
The lack of workstation motherboards seems to be a clue that AMD does not expect anyone to want a single processor Opteron (other than as a blade, I would guess).
And really, with the ease of making 2P Opteron systems (workstations here), why would you want to buy a single processor Opteron system? The extra price of a 2P system over a 1P would seem to be so small in this instance -- less than the top-of-the-line ATI video card you might put in it -- that it would be a shame to have your personal purchasing department restrict you to a 1P Opteron system.
I've always said it takes me a week to reconfigure/install/customize/tame a new computer -- if a 2P system lasts me a year longer than a 1P system my company is far ahead. (And so here I sit, composing posts while my project rebuilds :( ).
I think AMD and the motherboard manufacturers were on a different page on this one, not realizing how attractive a 2P workstation would be.