What does an exclamation point mean next to an e-mail address in an iCal invitation?
This means that the invite has not been sent.

You should check that the email address is correct, edit the event and try again. If the invite is sent and accepted a checkmark will appear here. If an invite has been sent but not yet responded to, a question mark will appear. This suggests that the invite has not been sent.

Make sure you are connected to the internet.

If all this fails, check that you have set up a card for yourself in the Address Book, as iCal uses this info to send to the invitee.