Now that I've had a couple spare minuts to look, nobody carries the low power ones. I read they were starting ti work their way into retail channls like a week or two ago, so for a sever part, it still could be a month or so.
OTOH, they do pull less power than a comparable Xeon, if not by too much .
As others have said, the cost effectivenes is definitely there, and while they are so-so for workstation stuff (much of which is P4-optomized), server apps, where little optomization goes on, tend to favor the Opterons in most cases, and while you can find server benches they lose in, the don't fare badly. AMD played their cards right, except for not forcing an AMD-specific trademark name on x86-64, I guess--but that' something for a few years own the road, anyway.