Yes I noticed that in my account yesterday

Yes, yesterday it appeared for the first time.

Not on mine :-(As I read it, it's only available if you verify your site via webmaster tools. But since Google forced me into splitting my webmasters account and my AdSense account (well they migrated my webmasters account without giving me a chance to stop it, to my then new googlemail-account) I'm left barehanded :-((

So I won't use this cloaking tool, since of course the pages which are behind a login are personalized, and so has the content presented to the Google Mediapartners-Bot be different to what the actual user will see. That's cloaking, right?

A human user who logs in will see the same as the Googlebot who logs in, so I don't think this is cloaking. Also, a "normal" user will get a password-protection and doesn't know the password because it hasn't been told to them, like a "normal" Googlebot will also get the password protection and doesn't get in...

Sorry, Phillipp, but I have to disagree, let me explain with a simplified example:
I log in: content says something like: Hello Michael, your last activities on this site have been..., these are the new pages in the field of your interests: content-a, link-a, contend-x, link-x

You log in: content says something like: Hello Phillipp, your last activities on this site have been..., these are the new pages in the field of your interests: content-b, link-b, contend-y, link-y

Google Mediapartners-Bot logs in: Hello Google-AdSense, here are the keywords I want you to display ads for: high paying keyword #1, high paying keyword #2, ...

You see that's cloaking, at least I as I understood it.

This is not working for the Search-Engine Google Bot of course, it still has to stay out...

Have you gotten this to work?

I think the Google Mediapartners-Bot hates forums. I have yet to get it to login successfully on any page in phpBB or vBulletin