This is an issue I have been struggling to understand for some time. I have recently had some time to sit down and actually figure out what was going on here, and have a full blog post with the gory details.
Basically, if you tighten down your user level mailfile ACL to Editor access for your users, the Out of Office functionality will be inconsistent between the BB device and what Lotus Notes displays. This is very bad when your users believe these OOO messages have been sending out when they actually have not been. It undermines trust in the BlackBerry device as a reliable tool.
I am working with RIM on this and hope to have them recreate the issue and log an SDR, for those who are interested in the details you can check out the full blog posting at: BlackBerry Enterprise Server on Domino Discussion: BES 4.x Out of Office Agent Integration Breaks when Mailfile owner set to Editor in ACL