If this would be true this section of the M$ Windows 7 Beta site wouldn't make sense at all.

Quoting Microsoft:

Quote:
What happens if I lose my product key?

If you lose your product key, just go back to the Download page and begin the installation process again. You’ll be asked to enter your Windows Live ID, and then you’ll see your product key you were originally issued. You can then use this product key to activate your software—you don't need to download the operating system again.

What’s a product key and why do I need one?

A product key is a long sequence of letters and numbers that’s used to activate the Windows 7 Beta and ensure your copy is genuine. (See Activating Windows: frequently asked questions.)

When you download the Beta you’ll be given a product key. (See Installing the Windows 7 Beta.) Be sure to use the product key to activate the Beta within 30 days, or the Beta’s functionality will be limited. Your product key will work for up to three separate computers, so if you want to install the Windows 7 Beta on another PC, you can re-use the same key.

But hey, it's only M$, so probably they mean that you could use any other key. 

Do you have any official source for the lifting of the activation limitations? I couldn't find any, but probably I was looking at the wrong sites.

Not quite,
what was lifted was the limitation of copies (originally 2.5 million, now unlimited).
What was not lifted was the need to have a personal license key handed out by M$. Which would have been a real surprise BTW.
So still: If someone is using a license key that he didn't get from M$, he/she is using a pirated version.

There's no difference between using a stolen license key for a final version or a stolen license key for a beta. And there's also no difference here between stolen, illegitimately published or originating in a keygenerator.