Previously, the OEM license was only for installing Windows on PCs intended for sale. The newest System Builder license includes a provision allowing you to install it on a system or a virtual machine for personal use. That's why they don't need to produce a consumer retail box any more.
Ah yeah, I should have stressed the download part. My local Amazon (UK) has an OEM version available on DVD which is ultimately probably how I'll have to do it and then rip it to an ISO. But it's 2013.. c'mon MS.