i would rather refer to it as always available rather then always on.

about the only reason for always on would be im.

but if one could get im, mail and mobile messaging to cooperate one would not so much need the net to be always on in the current sense.

say if one could add ones mobile phone number or similar to gtalk or some other xmpp server, and then one would instantly go to a "mobile" status if one was not logged on to said server.

or one could get the traffic routed to ones push mail service (ok, so that requires a kind of always on setup). still, doing im over the latter...

as i think about it, its more cultural then technical that seperates mail from im and sms. the latter ones are more about near real-time communication. hell, i know icq have/had a feature that a irc like, to the point of sending individual key presses between contacts (watching the text being typed was a interestingly strange experience), while mail is more about writing
"formal" sized texts more often then not...