Although the language is a bit cagey, it seems like at least parts of Windows 10 will be downloaded before the official availability date:

> Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly.

(from http://www.microsoft.com/en-us/windows/windows-10-faq > What happens when I reserve?).

I stand at least partially corrected, that certainly is some vague phrasing. It could be anything from installer files to the whole OS.

Likely to avoid overloading their servers on launch date by spreading out the downloads over the preceding two weeks or so.