A while back we hinted that we were working on implementing Guild banks into the game, which was a feature originally intended for The Burning Crusade that was postponed for a later patch: http://forums.wow-europe.com/thread.html?topicId=89460946&sid=1

A lot of work has gone underway since then and now we can finally see the light at the end of the tunnel. The Guild bank feature is still being worked on, however it is currently scheduled for release in the future patch 2.3.

[Update 30/08]

This is still under development, but currently the plans are as follows:
The Guild Master will have full control over the guild bank. He, or she, will be able to give permission to others to also have control over areas of the guild bank.
It will be possible to setup an area of the bank that everyone in the guild can donate to, but only restricted players could withdraw from. For example an area of the bank could be configured for herb donations and anyone in the guild could hand in herbs, but only the guild alchemists would be able to withdraw them.
This system should extend to currency too whereby different ranks within the guild would have access to different parts of the bank.

The guild bank will also have an online logging system on the website so that guilds will be able to track all the comings and goings from their bank.