We just expire accounts when we need the mailbox to continue to function, then disable them when we don't need the mailbox anymore.
I do have a question about the recipient policies though. Of course, we can receive mail for multiple domains with policies applied to the appropriate users... and we can set one of the other domains to primary so the users can send as that domain. However, there is no simple way to allow the users to choose which domain they are sending from on a particular email (e.g. in Outlook). Is there going to be any functionality in Ex2k7/Outlook 2007 to handle this without having to kluge something? My company has been on an acquisition spree and this is becoming more of an issue every day - would be a *really* useful thing to be able to do when necessary.