It appears from a article just posted by Mary Jo Foley that Microsoft will provide the Windows 10 upgrade for free for users of Windows 7, Windows 8 and Windows 8.1.​  It will also include Windows Phone 8.1 phones.  The catch seems to be that the upgrade will only be available for one year from the commercially available date of Windows 10, which is right now set to be Fall of 2015.

Microsoft to make Windows 10 free to Windows 7, 8.1 and Windows Phone 8.1 users – Mary Jo Foley ZDNet

More from Microsoft on Windows 10
http://news.microsoft.com/windows10story/