I have enabled selected users on my Exchange 2010 server to RECEIVE emails sent to more than one domain (e.g., email@example.com, firstname.lastname@example.org) by the obvious means of (a) appropriae DNS records, (b) configuring Exchange to accept mail for multiple domains, and (c) assigning alternate email addresses in AD.
(1) Inbound messages always bear the address of the PRIMARY domain, so users can't tell which it was sent to. (relatively minor issue)
(2) Users cannot SEND from multiple domains (MAJOR issue). In particular, it would be nice if (a) the address/domain SENT to was preserved, and (b) replies would be FROM that same adddress/domain.