From the Windows 10 page.
Great news! We will offer a free upgrade to Windows 10 for qualified new or existing Windows 7, Windows 8.1 and Windows Phone 8.1 devices that upgrade in the first year!
And even better: once a qualified Windows device is upgraded to Windows 10, we will continue to keep it up to date for the supported lifetime of the device, keeping it more secure, and introducing new features and functionality over time - for no additional charge.
Sign up with your email today, and we will send you more information about Windows 10 and the upgrade offer in the coming months.
The fine print at the bottom says:
It is our intent that most of these devices will qualify, but some hardware/software requirements apply and feature availability may vary by device. Devices must be connected to the internet and have Windows Update enabled. ISP fees may apply. Windows 7 SP1 and Windows 8.1 Update required. Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise, and Windows RT/RT 8.1. Active Software Assurance customers in volume licensing have the benefit to upgrade to Windows 10 enterprise offerings outside of this offer. We will be sharing more information and additional offer and support terms in coming months.