Two tips answering requests above:
Symprex Out-of-Office Extender is a Windows service that resets the OOF sender list on all mailboxes (or a selected group of mailboxes) every midnight for example. This ensures that a sender will get a reply the first time on any day that he/she sends an e-mail to someone who is out of the office (and still avoids mail loops).
http://www.symprex.com/products/oox
Somebody also asked how to let a helpdesk manage out-of-office replies for other users. Symprex Out-of-Office Manager lets you do that. The great thing is that the product uses a small service (which must run in the context of a user with receive as rights on mailbox stores) to read/update settings. This means that the actual user does not need any special permissions to read/update OOF for other users.
http://www.symprex.com/products/oom
These products are as far as I know currently being updated for Exchange 2007.