If you did a clean install of MacOS X 10.4, part of out of box experience is filling out a bunch of things about what your name and address is, where you work, what your e-mail is, etc.
CMD-Q bypasses that screen, and continues as normal without asking for irrelevant information.
Thanks to Jon Rentzsch for the hint.
I always thought that, apart from filling in your Username details, and Address Book entry, that it should pre-populate your default email account in Mail.app.
Maybe in Leopard?
Dave