I believe that just like Gmail, Gtalk (in Gmail), Facebook, and Twitter that it will become more popular and used instead of e-mail.
For me personally gtalk in gmail has replaced the need for having a dedicated instant messaging client like Aim, Yahoo, or MSN.
The biggest problem Google Wave faces right now is the fact that no one has Google Wave accounts. My guess is that in the future Google Wave will be able to communicate with e-mail clients but wont be able to take advantage of its fully feature set. This will allow for people to fully switch over to google wave as their main form of communication rather then typical e-mail. or there is the possibility that all gmail accounts could be turned into google wave account and then you have a huge user base.
Just a note here is a tutorial on using twitter in Google Wave:
http://daggle.com/add-twitter-google-wave-1424
-Oris-