Great news! The long-awaited ability to use standby Exchange clusters is finally completed testing and documentation with its addition to the Disaster Recovery Operations Guide: http://www.microsoft.com/technet/prodtechnol/exchange/guides/DROpsGuide/2493c2d6-618c-4c49-9cb1-fff556926707.mspx
This is one of those topics that has come up again and again over the years, and there was never any good (supported) solution for it. Well, over the past few months the Exchange team went through all the requisite testing and documentation, and now this is a fully-supported DR solution!
What does this mean? It means you can now recover your clustered Exchange 2003 EVS to a totally different cluster server (potentially in a different physical or AD site, with different IP, etc). See the updated DR Ops guide for more details.