I'm getting strange behavior with invitations I send from iCal to another .mac address. The invitations never show up (not in the invitee's inbox, nor in his iOS Calendar or iCal) and my iCal shows an exclamation point next to his address in the invitation:

What does this mean and how do I (or does he) fix it?
I'm using iCloud and he is using MobileMe, but I find it hard to be believe that should prevent me from inviting him (or even notifying him).