You cannot purchase a retail copy of Windows 7 at this point, officially. The last day of retail availability was October 30th, 2013. System builders can still include the operating system in their PCs, however, until October 31st, 2014.
Windows 7 Professional is the exception.
The Windows lifecycle website claims that OEMs can include Professional in PCs until a to-be-announced date. That date will be at least one year after whenever they decide to announce it. As of February 16th, the date is still listed as "Not yet established".
I should note that Volume Licensing customers have downgrade rights and installation media available for the two versions prior to whatever is current. In short, they have their own timeline.
Basically, we know that preinstalled Windows 7 Professional availability is on a countdown timer. We know that timer is at least one year long. We do not know how much longer than a year it will be. We also do not know when the announcement will be made and thus, when the timer will start ticking.
The Ars Technica article claims that this Windows 7 Professional OEM extension is for business users. That said, a fair amount of those users are on volume licensing. Another possibility is that Microsoft wants to bridge the gap between Windows 7 and the rumored "Windows 9" for enthusiasts. "Threshold", as it is codenamed, is supposed to address users who are primarily in the desktop interface. Professional would give them devices to purchase until then, without the general public purchasing a cheap Windows 7 machine and intending to use it for a decade (potentially beyond Windows 7's EOL in 2020).
Windows 7 Home Basic, Home Premium, and Ultimate will no longer be preinstalled in PCs on October 31st, 2014. Windows 7 Professional will be available for some unannounced time afterward.