I actually saw the same thing on amazon today and I'm getting it right now. It's downloading, says it'll take five or so hours. Downloading through amazon's downloader but says you need Steam to "Install the game and play online". Also in that table of requirements it says 20 GB too. Will try to remember to let you know how it goes.
*Edit* Took forever to download, forever to install, and another forever in steam, got it at around 6:00pm monday, it's 1:06AM Tuesday and it's at 99%.
As for how it works, you download amazon's downloader, it downloads the game, then extracts it. You enter your "CD" key and steam then installs it. Then for whatever reason it started at like 84% and installed it again.
All in all if I had to do it again I'd get the disc version so I could play it earlier with out the hassle.
[This message has been edited by Zakkleburg (edited 06-21-2011 @ 01:08 AM).]