I know one of the issues (which I am working on now - setting up a backup exchange server) is keeping the "offline" server up to date.
If exchange sees another server with the same name on the network, it's not pretty. The backup exchange server also sees "this isn't my database" and won't mount the IS if you copy over the information.

I havn't found much helpful information for exchange as of yet. If you run into something, please let me know.