Yes, several play by email systems (also known as PBEM) have been
used.  One is available from Dewey Barich (barich@tam2000.tamu.edu),
email him for information, or if you'd like to get involved in
a PBEM game.  He also hosts the Annual Axis & Allies Internet
Tournament, which goes on for most of the year.  There will be an
announcement posted to rec.games.board in the fall to begin
recruiting players for the tournament.

There is a new contact list called "The Axis & Allies Playing Field",
and it gets players together for PBEM games.  It has the largest
list for A&A of the groups listed.  Contact the following people
by email if you are interested:

Normal Axis & Allies: Chris Goldfarb (goldfarb@texas.net)
                      Richard Willis (willsr@caedm.et.byu.edu)

Variants & Expansion Set Games:
                      Morten Steinvik (mortenst@stud.idb.hist.no)
                      Dag Rune Kvittem (dagrk@stud.idb.hist.no)

* Also, Eric Pass (epass@nyx.net) maintains the PBEM Gamers
Directory, which attempts to get people together to play games
by email.  Contact him if you wish to be on the list, and make
sure you mention Axis & Allies, since his list contains other
games as well.  A&A has the largest membership on his list!