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Personally I dislike this option because I don't like doing something that could get obliterated by an Apple system update. This may be someone else's preferred method though so I will put it here for the record. Note that I discovered this solution when trying to figure this out myself and came across the thread http://lists.apple.com/archives/macos-x-server/2008/Nov/msg00210msg00241.html.
Add the following XML snippet to /System/Library/LaunchDaemons/org.apache.httpd.plist.
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