As is usual with Exchange rollup updates this one includes important fixes which means you should apply relatively quickly (of course after testing in your labs). Especially one of the fixes in this RU is interesting as its been present since Exchange 2007 RTM. It’s this one I have in mind. As you can read it revolves around free/busy requests between Exchange forests. So back since Exchange 2007 RTM, you had to be able to resolve the internal URL configured for the Exchange Web services (EWS) virtual directory, which could be a problem in scenarios where the lookups were planned to occur over the Internet or had Exchange deployments with an Internet-facing site and one or more non-Internet facing sites (proxy sites).
Well with the fix included in RU6, the remote forest just needs to be able to reach the external URL of the EWS virtual directory in the Internet-facing site and thins just works.
I know Greg Taylor pushed for this change to occur since Exchange 2007 RTM so he will probably be out partying tonight!
Technology Architect/Writer/MS Vendor
MCM: Exchange Server | MVP: Exchange Architecture