Although not "officially supported" you can get SIP to go directly from CM5.x to the OCS Mediation Server. I did away with our SES over a year ago since we are not using SIP endpoints.
If you use OCS for more than just voicemail You need to carefully evaluate items such as survivability and E911 functionality. These are 2 items that can get you into hot water and open your company up to liability issues. You better have a damn good Microsoft Partner to use for the OCS implementation.
You may also want to get the details on Avaya's UC products and how they integrate. Voice, VM, IM, and Conferencing through one integrated client.