"It is a conundrum. If I were to hack into Microsoft and obtain the source to Windows 8, release it on github under the GPL - it seems highly unlikely that would be honored. Although if even one single person downloaded it under GPL then technically they should be able to distribute their own version under GPL."

While I think there is a lot of legal subtlety at play in the nv3d case, I think your case is a lot more clear cut. You never had the legal authority to make the Windows 8 source code GPL in the first place, so the code isn't GPL, no matter what the README you attached to it says and people continuing to distribute it would be in clear violation of copyright.

Your Windows 8 example is pretty clear cut, sort of like if I steal your car and resell it to someone else. Just because they bought the car doesn't mean they own it, it still belongs to the original owner though the person who bought it clearly has a legal case for restitution against the person who sold it to them without owning it.

Yeah, it probably wouldn't be honored unless the company did something to indicate they sanctioned your actions. Your second scenario, in which the company leaves it up for a year, may go a little ways towards creating apparent authority. However, if you had no prior connection to Microsoft, the court would probably say a reasonable person would not believe you had the authority. Even if you were a Microsoft employee, because it's Windows 8 in particular, the bar would be pretty high for showing that a reasonable person would actually trust the authenticity of this open source license.

"... If you knowingly left it up for a year, though, that would certainly be a different situation."

It would be different but it would still be utterly unlike what jarrett describes.