Set Out of Office Reply from an Adminstrator Login in Exchange
In the AD tools, make sure the "Advanced Options" are enabled.
Then you can change mailbox access rights to give You, the admin. the ability to login to the mailbox.
Then login to the mailbox (Outlook Web Access is particularly useful here)
And create the Out of office message.
Sources:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&threadm=3CsR7.886%24TI5.110923%40news11-gui.server.ntli.net&rnum=1&prev=/groups%3Fq%3Dopen%2Bmail%2Bbox%2Badministrator%2Bexchange%2B2000%26ie%3DUTF-8%26hl%3Den%26btnG%3DGoogle%2BSearch
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