A kindly NeoGAF user was looking through the Windows Compatibility website when they stumbled upon this. (Screenshot in case it is taken down) The big question that needs to be answered is how does a program get listed on the Windows Compatibility website in the first place? What kind of testing, if any, has to happen for it to be listed? Once we know that, some credence may be given to the rumor.
As it is right now, Red Dead Redemption has passed for Windows 8, 8.1, and 7. There are several ways to be approved for compatibility:
1. The product has passed Windows certification requirements and received a logo, which indicates it has met Microsoft testing requirements for compatibility with either Windows 8.1, Windows RT 8.1, Windows 8, Windows RT and/or Windows 7.That reveals one incredibly important thing. At some point either Micrsoft or the program developer (Rockstar in this case) must have conducted some kind of test to validate the program's compatibility with Windows for it to be listed on the website. That means, that the likelihood of a Red Dead Redemption PC port existing seems very good.
2. The app publisher or device manufacturer stated this products works with Windows 8.1, Windows RT 8.1, Windows 8, Windows RT and/or Windows 7. Please note that these products have not undergone certification tests and may or may not be fully supported by the manufacturer. You should visit the manufacturer’s website for more information.
3. Windows 8.1 is designed with compatibility in mind. If an app or device ran on Windows 8, our goal is that it should run on Windows 8.1. During Windows development, we test most of the popular products and we are providing compatibility statements based on this testing. For products we have not tested, compatibility statements for Windows 8.1 are being provided based on the product's compatibility status for Windows 8.
Is this definitive evidence of a Red Dead Redemption pc port? No. But, it is fairly interesting nonetheless and seems especially convincing considering it is on an official Microsoft website, which the only way to be listed is either by the original program developer or Microsoft.
However, of course, take this with a grain of salt and dampen your hopes until (if) Rockstar does make an official announcement.