in-game mail access is being considered for the armory mobile app. It is of course far from being confirmed -- but still, in theory, if it were implemented, it could be handy, allowing people on the go to transfer items between bank alts, manage consumable distribution between characters and more.
Vaneras also said that Blizzard is always researching new potential additions to their web and mobile services as well, which actually piques my interest even more. I'd personally love to see guild chat and in-game chat channels added to the mobile and web services. If I could use a messenger program to stay hooked in to my guild and my friends list even when I'm not playing, I'd be willing to throw a few extra bucks a month Blizzard's way.