There's the option to share forums with another guild, and you can cooperate via members (basically you support members of the other alliance who are in your neighborhood and help others as friends). I don't think there's more that an "alliance" can provide. Probably there will be more features when Guild Wars are implemented but that's definitely in the future.
Some guilds, like mine, have alliances which are formed by creating mutual friendships between the players and a shared forum space. This creates a pact of non-aggression, more trade resources (albeit with forge point costs), increased polishing and motivations, and higher chance of obtaining blueprints. Works alright, but hard to maintain with fluid guilds. Best done with guilds which have stable and established groups of long term players.