Greg,
The following topic covers the process for installing a rollup on DAGs.
http://technet.microsoft.com/en-us/library/ee861125.aspx
Sheldon,
Interim Updates are built for a specific rollup. They are a temporary solution provided by Microsoft Support when you have a case open with them for a fix which will eventually be available in an Update rollup. But because they are specific to the rollup, any Interim Updates you have are not suitable for Update Rollup 2.
The following document covers the steps you need to take if you have Interim Updates on your system. http://technet.microsoft.com/en-us/library/ee221147(EXCHG.80).aspx. It is an Exchange 2007 topic but the principle is the same for Exchange 2010. We have an Exchange 2010 version of this available soon.