Hi,
ProxyAddresses is an attribute that Exchange add to Active Directory and is also maintained by Exchange, whenever you add an email address to a mailbox this attribute get's modified. The SIP URI belongs to a different attribute called msRTCSIP-PrimaryUserAddress. As Mike indicated you want to make sure that the SIP URI of the user is added as an email address to the user's mailbox. If the user's SIP Address is indentical to the user email address then this will already be the case and you should have clean integration between Outlook and Communicator
Here is some more information for you:
Common Errors: How do I get rid of the red BANG?
http://communicatorteam.com/archive/2008/08/20/316.aspx
How to programmatically set the proxyaddresses attribute?
http://www.microsoft.com/technet/scriptcenter/resources/qanda/may05/hey0510.mspx
The OCS Multiple Forest Deployement guide mentions the following:
http://www.microsoft.com/downloads/details.aspx?FamilyId=1D7CF1E6-6770-422F-B744-E1764F5666AE&displaylang=en
Important The primary user address (msRTCSIP-PrimaryUserAddress) of the contact must be added to the list of proxy addresses in the ProxyAddresses attribute of the mailbox object. This is required for integration with Outlook and other Microsoft Office products.
Hope this helps...
Sincerely,
Tonino Bruno