What are your message record retention policies? Some customers I've worked with are very hesitant to store voicemail and email on the same system. They are fearful that this would provide them with a legal or retention problem. If you have obligations to retain voicemail messages then it doesn't matter where these messages are stored. Another perspective would be if you didn't retain them prior to Unified Messaging with Exchange then storing them on Exchange doesn't change that. Basically - your policy stays the same.

We did work with a law firm to study this problem and here is their whitepaper: http://www.microsoft.com/exchange/evaluation/unifiedmessaging/dataretentionwp.mspx

Exchange does problem you with some tools to setup compliance and retention policies around Unified Messaging. You can setup policies that prevent voicemail from leaving campus via a transport rule. Journaling voicemail can be setup to be retained or skipped for the campus.

Set-TransportConfig -VoicemailJournalingEnabled $False

How about disclaimers? You can setup them up for UM this way:

Set-UMMailboxPolicy -identity MyUMMailboxPolicy -VoiceMailText "Voicemail is retained at XYZ University for 30 days."