What’s your DR plan for Mail System?

hi all,
Just want to do a survey on the DR plan for the *critial* mail system.
Actually, it’s really depends on the budget for the technology to be adopted.
* Form a application server cluster?
* plug the mailbox storage to SAN?
* Set up a two mail systems on two office location and sync the transaction logs through WAN?
* Backup to tape
Is there any different plans for Exchange vs Notes.