I’ve repeatedly encountered a problem when sending iCal invitations to people. When I create an iCal event, add the invitees, and press Send, iCal sends the invitation from my MobileMe account instead of my work account. 

Is there a way I can define the mailbox that iCal uses to send out the invitation, or do I have to copy and paste the invitation into a message in my work email account?
Via the internet

You can fix the problem quite simply: open Mail’s Composing preference pane, and configure the Send New Message From pop-up menu so that it contains the address from which you want to send iCal events.