It's been almost a full year since I last saw this brought up, and we still haven't seen anything from Blizzard on it. Eric over on WoW LJ brings up the issue of tracking alts within a guild. As we found out last time, most people just use guild notes (or helpful addons) to keep track of whose alt is whose inside a guild, and it works OK -- assuming people are up front about when they're logging in on another toon.
But still, it seems like an official interface would work much better. And especially considering that Blizzard is actually going to start encouraging alts as a part of the gameplay (you have to have a character at least 55 on the account to start up a Death Knight, and there are rumors that alts will benefit from reputation grinds as well), it seems like they might be interested in making the connection between characters on the account a little more obvious.
And as we said last time, there are benefits to the guild as well -- you never know when that lowbie leveling through Westfall might actually have a Death Knight ready to tank for you. I'm still not sure about marking alts in the Armory, but if you join up to a guild, I'd think guilds should have the right to know, officially, who's an alt of whom.