According to Tech ARP, a site known for Microsoft leaks, those buying Vista PCs between July 1st and January 31st from could score a free upgrade to Windows 7.
The offer, known as Microsoft's Technical Guarantee Program, seems OEM-specific, meaning that it's only valid if you buy a PC through somebody like Dell. The other catch is that the PC manufacturer can change dates of Microsoft's offer at will, shortening the window for free upgrades.
All of this information should still be taken as rumour, but those looking to upgrade their computers can probably feel safe aiming for July, even if Windows 7 isn't out yet. [Tech ARP via Lifehacker]