IMPORTANT : Exchange 2010 RTM article can be found here :
http://www.pro-exchange.be/blogs/exchange2010/archive/2009/11/16/installation-of-ocs-2007-r2-web-service-provider-for-exchange-2010.aspx

First of all, you have to download the bits here:
http://www.microsoft.com/downloads/details.aspx?familyid=CA107AB1-63C8-4C6A-816D-17961393D2B8&displaylang=en

Then install the MSI file on the Exchange 2010 CAS Server(s), this will extract 4 installation files into a directory on the server.

Updated : Hotfixes Available:

• Also download a hotfix for the OCS 2007 R2 Web Service Provider
  http://www.microsoft.com/downloads/en/details.aspx?FamilyID=45C94403-39FA-44D3-BE23-07F25A2D25C7
• Unified Communications Managed API 2.0 Redist (64 Bit) Hotfix KB 2282949
  http://www.microsoft.com/downloads/en/details.aspx?FamilyID=1F565A42-71D2-4FBD-8AE0-4B179E8F02AB
  (Note that you have to download the UcmaRedist.msi hotfix if you are running Windows 2008 R2 )

Open the installation folder

You must install the files in the following order:

• vcredist_x64.exe
• UcmaRedist.msi
• UcmaRedist.msp
• CWAOWASSP.msi

Now you need to get Certificate Information for Exchange
Note : The Client Access Server must be configured to use a certificate which is trusted by OCS 2007 R2
(This means that it requires a none self singed certificate!)

You must get the following information for the Certificate on the Exchange 2010 CAS Server :

• The Certificate Issuer
• The Certificate Thumbprint

Type Get-ExchangeCertificate and use the format list output to view all information (Get-ExchangeCertificate | fl)
Note : You need the Certificate that is configured for IIS

Now you can configure all settings at once with one command:

Set-OwaVirtualDirectory -Identity "XFE\owa (Default Web Site)" -InstantMessagingCertificateThumbprint 19B735DA5F63CC0FF3221962749CFF7D18B014F1 -InstantMessagingServerName ocs.delimon.be -InstantMessagingType OCS -InstantMessagingEnabled $True

Now you should restart the IIS Server

If the Windows Firewall is enabled on a Client Access Server running Windows Server 2008, you may need to add an exception to allow OCS 2007 R2 to communicate with the Client Access Server

On the left side of the Windows Firewall window, click “Allow a program through Windows Firewall”
Add the IIS Worker Process (“C:\Windows\System32\inetsrv\w3wp.exe”)

Exchange 2010 CAS Configuration is completed
Now OCS Configuration on the Front-End server must be configured

The Exchange Server 2010 Outlook Web Access IM Integration component is implemented as an OCS 2007 end-point. In order for the integration component to be able to sign-in to OCS 2007 R2 it is necessary to configure the OCS 2007 R2 server to trust the Exchange Client Access Server. This is configured by adding the Exchange Client Access Server as a trusted server on the OCS 2007 R2

Open the Front-End Properties of the Pool in Office Communications Server 2007 R2 management console and open the Host Authorization Tab

Add the Exchange server as an Authorized Host:

• Add the FQDN of the Exchange 2010 CAS Server
• Select Throttle As Server
• Select Treat as Authenticated

You should see an entry similar to this one

Now that you have configured the OCS Front-End Server you need to restart the Front-End Service on the OCS Server

Now you are done configuring the integration :)

Presence integration in Outlook Web App screenshot (Note the Updated square presence icons)