Microsoft has started loading Win 10 on to computers today.  They are not notifying users its ready for install, just downloading.
If you signed up for the Win10 upgrade, check your Operating System drive for a "$Windows.~BT" folder with today's date. 
 
My Windows 8.1 Pro 64 bit OS has the update folder.  I'm not upgrading my Win 7 OS so I don't know if its both Win 7 and Win 8.1 or only Win 8.1 systems.
 

Nothing on my system yet, but I have Automatic Updates disabled so as to avoid background downloads while I'm doing audio stuff. 

Nothing happening in the UK at my end it seems, I guess they really are actually going to stagger it over a a few weeks.

https://www.reddit.com/r/...e_download_windows_10/
You can force start it. :D

With a 404?

I downloaded it and installed in manually.  If you select ISO to make a DVD, you have to enter a serial number.  If you choose Upgrade this computer, it is painless.
 
microsoft.com   /en-us/software-download/windows10

StaringIntoNothing
If you select ISO to make a DVD, you have to enter a serial number. 

I do want to make a DVD. Can I assume the requested serial number is that of my previous Windows installation, or do I have to somehow acquire a new S/N?
 

I did not know the answer last night so I switched back to the Upgrade button.  I have seen nothing on the Microsoft site but I have seen in other IT forums today that your current O/S serial will work.  I consider those posts rumors.  The ISO downloaded fast for me so try it and see if it works.  Best I can say at such an early stage.

Yes, your product ID number found in the control panel under Systems is the number to enter. I saw somewhere amongst all the rumors that if you skip putting in the product ID key that windows 10 will automatically search and find the number itself but I don't know if I trust that logic or not!

BTW, if you want to dual boot, you have to BUY the windows 10 license. You can only get the free upgrade if you overwrite the existing OS.