BTW, the IETF just published International Email stuff as Proposed.
And, at brief look at it, yes, it does seem like they use UTF-8 on the wire with fallback to some ASCII address if the wire doesn't support the UTF8 extension.
For White Pages applications, one could see it desirable to have a UTF-8 version of 'mail'. Fallback, if one considers it, looks to be messy.
Probably would be good for some email MTA and MUA developers to sort out what want in the directory and put forth an I-D here (with the help of some directory folks).
-- Kurt