I think it was an unboxing video which answered some of those questions for me:
1)there is a pull-down menu letting you pick how to connect - different wifi locations are listed there alongside the 3g card
2)a brief hold of the power button logs you out, iirc, and you can log in as a different user or as a guest. to power off or reboot it takes another brief holding of the power button. I think the second power button press is required since when you close the machine it goes to sleep where the battery probably lasts for days, so most of the time you would just log out and close up.
3)when you start up it shows you a graphical log-in screen, guest as an option after any accounts you created
I do not know about sharing the google space allotment or the threads about going to multiple gmail acounts with a single chromebook account.
Last edited by Ben; 05-20-2011 at 02:51 PM.
Reason: forgot about the first question, added answer 3