You'd have to use a program like InsomniacX, as the computer will go to sleep when closed regularly. I think there is probably a setting to not use bubbles in iChat, but I use Adium.
Per the first. Closed lid = computer in sleep mode = no internet = ichat disconnects. If you want iChat to stay connected then (as someone said) you'll need another program so that your computer stays on while the lid is closed. IMO not a good idea but to each his own.