...And I wouldn't find it surprising if it truly is "flying off the shelf" compared to prior things. And I think many people will dig up the money to upgrade their OS if it's affordable and relatively guaranteed to work. They'll do that before they go buy an unnecessary fancy sweater, or a couple DVD's, for example.
Microsoft fairly aggressively had cheap pre-orders. Their $30 deal for students is hugely popular. Etc.
Lots of people indeed did the Release Candidate and word spread, so people looked into things earlier than usual. People have largely learned it's backwards compatible and better than Vista in advance.
On the other hand I think something like Vista was known to still be buggy and having lots of issues around its release date. It was also overpriced with people being asked to step into the unknown, and often upgrade their hardware in the process. No thank you.
There aren't backwards compatibility issues or driver
issues as happened from XP to Vista. Many people have a perception of Vista as "broke" so they're willing to "fix it" on a machine they use hours per day. XP to Vista was more in the category of "if it ain't broke don't fix it." This is the first time they haven't done things likely to "break" prior hardware, etc. when someone upgrades. Some prior things could render a printer or scanner obsolete overnight and without warning.
And I'd expect computers outside the U.S. are increasingly more widespread, with lots of people ready to improve from XP, and with their own good deals available.
Sure, businesses might take months (or years) to let things settle in before they transition, but from the consumer perspective, Windows 7 was set to move quickly.
And as far as any retailers' numbers, or the guy unable to sell his copy for $50.... I'd expect they're selling more downloads than usual now, especially due to things like that student deal. And most consumers are going to be hesitant buying from some random guy selling a box for $50 -- they'll assume it's stolen, or fake, or a copy not for resale, or a boxed beta, etc., and there's no way they can return it if they have a problem. I know I wouldn't go over around maybe $50 for a boxed thing listed someplace like Craigslist. No surprise there.