For what its worth, I stopped by the American Embassy my last trip to Russia last month.....the person in front of me was asking the same question and they were told that they could get themselves registered in the post office and they were given the form to fill out. (doesn't mean they were told correctly).

I totally agree with the posters that say to get registered especially with the Georgia conflict. In July I was being yelled at when leaving for not having printouts of my tickets even though I obviously had my boarding pass (How do you think I got my boarding pass if I didn't have a ticket - not to mention how did I get to Russia in the first place). After I said 5 times that I had an electronic ticket the man gave my passport back to the immigration woman and she finally stamped it. It was a fairly ugly scene and even the Russians in my line moved to other lines to avoid this booth (think it was dumb luck that the guy was there as I think the woman was asking him a question about the previous person going through)....It was somewhat funny when I looked back as literally no one was going through that line after me even though the other 6 or 7 lines had rows 10 deep. Given the current state of affairs I think this would have had the potential to get very bad, and no reason to actually give a reason for problems.

Aside from the one incident the changes have been very good as far as I'm concerned - I stay at hotels in Russia and they are now responsible for your registration and you don't have to carry anything around (less reason for police to stop you also, though I haven't had much trouble in the years I've been going to Russia).