As always, good info from Chris. Another option is
SmartSIP, which can add your Active Directory display names to the caller ID that is presented to your Cisco phones. It can also do things like pass your 4 digit caller ID to Cisco but a full 10 digit caller ID to the PSTN if it's not possible to get your Cisco config to do what you need. Plus, when you're ready, you can reuse your Cisco phones as OCS endpoints instead of buying new hardware to move your users to OCS Enterprise Voice.
Mike Stacy | Evangelyze Communications | http://www.evangelyze.net/cs/blogs/mike