Wow..... the high availabilty changes in Exchange 2010 are a game changer. If it all
works as well as described every Exchange shop in the world will be redundant after
adding a second server. I remember asking about mailbox replication at the MVP
summit a few years ago but they weren't there yet. I'm glad to see they kept on that
direction and hope this turns out to be a solid feature. Paul has a good rundown of the
details involved:

http://www.robichaux.net/blog/2009/04/exchange-2010-availability.php