RIM have officially added support for Exchange 2010 to BlackBerry Enterprise Server 5. Following hot on the heels of SP1 for BES 5 (http://blog.brightpointuk.co.uk/service-pack-1-bes-5-released), both RIM and Microsoft have released updates for BES 5 and Exchange 2010 to enable compatibility.
The pre-prequisites are therefore:

• BES 5.0.1 MR1 (ie BES 5 with Service Pack 1 and Maintenance Release 1)
• Rollup Update 1 for Exchange 2010 - http://www.microsoft.com/downloads/details.aspx?FamilyID=371add31-d7a0-4...
• Microsoft MAPI CDO 1.2.1 - http://www.microsoft.com/downloads/details.aspx?familyid=e17e7f31-079a-4...

For detailed information on how to configure Exchange 2007 / 2010 for a BES deployment, read this article - http://blog.brightpointuk.co.uk/bes-41x-50-exchange-2007-pre-requisites

The Exchange 2010 area of the BlackBerry web site can be found here - http://na.blackberry.com/eng/services/server/exchange/2010support.jsp