It... is and isn't. I literally just bought one yesterday and installed it on a laptop I got (that normally has Vista).
Basically,
go to the website, put in your info, indicate you bought a Win7 system around... Oct/Sept. You get a coupon code.
Then use the Win8 Upgrade Assistant, it checks your stuff, and then offers to 'upgrade' for $40. You put in coupon, it changes to $15. You then get an email with a license key.
EDIT: There is a bug with the Win8 Upgrade Assistant, in which it will tell you that "Windows 8 is not available to download in your region". This is resolved by running the Win8 Upgrade Assistant program in WinXP SP3 compatibility mode. It's stupid.
You can then START to install Win8 (Its called Create Media option, not Install Now), at which point it will allow you to download and create an ISO, which you can then burn to DVD, or put on flash drive.
Boot from DVD/USB, and you can do an utterly clean install of Win8. No installing previous OS, no in-windows upgrade.
I will likely be buying 3 or so more in a day or two, just to have a few licenses. I'm not a huge fan of Win8, but I can't deny that it's a good OS. Just frustrating to use at first. And for $15, how do you pass that up? As someone who sells computers and refurbishes, that's like adding $80 value to a computer for $15.