Microsoft has announced the availability of downloadable versions of Windows 7 and destined for machines that do not have any optical drive like Netbooks or ultra portables.
The online-only versions are apparently only available in the US and are designed to work straight from a USB stick (or any external storage device). For the time being, this option is only available in the US and the product page offers both Home Premium, Professional, and Ultimate products.
To make things even more complicated, they are upgrade versions only, which means that you will need to have either Windows XP or Vista on the netbooks which rules out any transition from Linux (ed: it seems that they now offer the full version as well).
You will possibly need to change the boot setup in your computer BIOS but otherwise there's not much that needs to be done. It is surprising as well that Microsoft has not pushed the Windows 7 Home Basic or Starter Edition as downloadable versions.
Prospective customers can download the Home Premium version (upgrade) for $120 and the full version for $200, the Ultimate upgrade and full versions will cost $220 and $320 while you will have to fork out $200 and $300 for Windows 7 Pro upgrade and full version respectively.
Obviously, it is not ideal since you can't do a clean install (although the overwhelming majority of netbooks are on Windows XP which means that the installation will be a cleaned out anyway). The file being offered is an executable file, ot an ISO one.
Save yourself the hassle of downloading and saving a multi gigabyte file by following our nice step by step guide which shows you how to build your own USB Windows 7 boot drive for around half the price (and time) that it will cost you to download an official version by Microsoft.