It's a bit interesting that the final ISO was leaked on the 23rd, a day before the royalty OEMs got them (they are supposed to be the first to get Windows). So... I think that the leaks are coming from within Microsoft.
After reading this MSDN blog post
, it sounds like Windows 7 was made available to the employees with keys and all. That also explains how the German Microsoft employee who published the hashes knew what they were. And with probably tens of thousands of employees who were given access, it's probably not hard to find just one rogue employee who's willing to sneak out a copy. So while everyone was focusing on the July 24 release-to-OEMs date, the real leak was probably coming from Redmond itself.
And I'm guessing that the subsequent OEM ISOs that came a couple of days later were probably leaked by an employee of a royalty OEM who got the stuff on July 24 (and not by a MSFT employee, as I think most MSFT employees who are not a part of the Windows team would not have access to the OEM ISOs).
As for why nobody has leaked a Home Premium or Professional ISO, my best guess is that the people who have access to this are all using Ultimate (after all, if you're a MSFT employee with free Windows keys, would you even think about the lower editions?).
Now, while I think that the leaks are coming from MSFT, I am not saying that MSFT condones this. It's possible (though highly, highly unlikely) that these are intentional, but the most likely scenario is that some rogue employee downloaded it from Microsoft's internal MSDN-like thing, put it on a flash drive, and took it home and surreptitiously uploaded it using his home Internet connection.