See webfinger.

Email is a federated system just like status.net, rstat.us etc. We can all have our own clients, servers, and still interact, communicate, and play well with every other email server.

Using the same means of locating a person seems very very reasonable from both a technical aspect (it works for them just fine) and from the viewpoint that people already understand that an email address represents somebody's online presence and this is how you can communicate with them. Concept familiarity is extremely important for adoption, so that should be considered.