I've been shooting with the 5D Mark II for a solid week now, and haven't seen any black dots. I even aimed it straight at the Moon last night, using Live View, and none came up. Could it be that my loaner is some magic version of the camera?
As I've been poking around the web reading up on the black dots "catastrophe" it seems to me that it's yet another problem caused by extreme pixel peeping, as much as anything. I certainly wouldn't be too worried about it, but most of the "sky is falling" comments are coming from those who are reacting to 200% crops from other people.
A lot of the more concerned people have yet to actually use the camera themselves. Some commenters are people who probably never had any intention of getting their own 5D Mark II to begin with. But that's life on the internet, right? I'm just saying, be careful what you read out there.
If this is indeed a huge problem — and I have to reiterate, that after a week of snapping frames, many at high ISOs, I have not seen this — then it'll be corrected by Canon. For some balance in your research, read this explanation by Ken Rockwell. Say what you will about him, Ken knows his stuff.
I guess I'm just lucky, or I haven't been trying hard enough to reproduce the problem. I've also been shooting in RAW, so perhaps that's the answer, though I took a few hundred jpeg shots one day without seeing it.
It'll blow over soon, so everyone can get back to arguing about high-ISO performance, lines of resolution, chromatic aberration statistics, and the usual.