A few comments on Gmail:
1. If you're a terrorist and use Gmail, what kind of ads do you see? My wife (Emily) rattled off a few. Be a good "future" spot in Wired -- albeit somewhat dark. Since AdWord advertisers often take keywords and insert them into their displayed ad text, it ought not to be hard to trick Gmail into displaying ads that look something like:
eShopper -- Yellow Cake Uranium at lowest prices!!
2. Their secret weapon is spam filtering. Google will adapt the best standards for both sending-service and author identification, but they'll also implement the practices I've advocated for years (to little applause I might add! :-):
- reputation management of authenticated sending services
- differential filtering based on managed reputations of authenticated sending services
and they'll implement practices others have tried -- including collaborative spam identification.
Google will do collaborative spam identification better than anyone -- they have the knowledge and ability to leverage knowledge about the account owner, semantic ranking of content (junk spam has low semantic rankings), advertiser links (if advertiser links switch from classical music to discount viagra ...), and to combine that knowledge with subscriber identification of spam. When 10 high reputation subscribers mark very similar messages as spam, Google can remove them all.
Put all of these together with the current exponential decay of traditional email (almost 30-40% of my filtered email is now nonsense-spam) and Gmail may be the only messaging service left standing.
Historians will note that anonymity on the net ended the day Gmail recognized that reading email to generate ad links also enabled spam filtering to work.