That domain does not have to be the server they use for microblogging or whathaveyou. It shouldn't be, honestly. You can host your identity anywhere and point to the microblogging server using a link.
So, I can be firstname.lastname@example.org, as familiar as an email address which is REALLY important for usability; the UI can drop the domain if you think it is ugly or hinders readability, and we go on from there.
This is what webfinger + xrd gives you: http://code.google.com/p/webfinger/wiki/WebFingerProtocol
It's already used by status.net, rstat.us, etc etc. tent.io ignores it and wants to reinvent everything.
Now I can switch microblogging sites if I want by just pointing to a new one. But, just like changing your name is a hassle in real-life, changing where your identity is held is also hard. Using DNS and having your own server for at least your identity are ideal.