Holy ancient post, Batman!
Anyway, just to help out anyone who comes in here in the future ...
If you're using Exchange and Entourage 2008, you can enable out-of-office messages in Entourage's Tools menu. If you're using Exchange 2007 or later, there are more out-of-office options available.
If you're using Exchange and Entourage 2004, you can go into Outlook Web Access to enable out-of-office. (Of course, you can do this if you're using Entourage 2008 as well, but I prefer to do it within Entourage.)
If you're not using Exchange, then you'll need to leave your Mac at home, and leave Entourage running. Then you can set up a rule to automatically reply to any message with a response to let people know that you're gone.
A point of out-of-office etiquette: whatever version of Entourage you use and whatever server you use, you should make sure that you unsubscribe from mailing lists before you leave. This prevents you from spamming mailing lists with your out-of-office replies. Just resubscribe when you get back. Many mailing lists will automatically unsubscribe you if they start getting out-of-office messages from you, so you might as well avoid getting the list admins and the other members of the mailing list angry at you.
Regards,
Nadyne.