Thought i'd post it here:
This is what i've managed to find out about the (still disabled) Guild banks
http://img101.imageshack.us/my.php?image=bank1zj4.jpg
This image shows the first guild bank tab (seems i wasn't quite triggering it right for this to show instead of the "buy tab" option).
At the bottom you can see the withdraw/deposit money buttons, and the tabs for the money and item log windows
At the top of the window is what appears to be the Guild Crest, presumably to be the same as the tabard design, however at the moment it appears incomplete and the design is controlled by the 6 floating buttons at the very top.
http://img84.imageshack.us/my.php?image=bank2cd9.jpg
The tabs appear to be purchasable simply by clicking the button, (although it doesnt work at the moment). The first tab of 98 slots appears to be a mere 1g.
http://img101.imageshack.us/my.php?image=bank3jp2.jpg
This one should be fairly obvious - how much gold would you like to deposit/withdraw?
http://img157.imageshack.us/my.php?image=bank4py6.jpg
No idea exactly who can buy new slots, but presumably its to do with the "manage tabs" option in the last screenie.
http://img157.imageshack.us/my.php?image=bank5ce0.jpg
When you create a tab you are invited to select a name and an icon. The icon window is not functioning correctly in the screenshot, but from the code i would guess it works pretty much the same way as the macro create window.
http://img132.imageshack.us/my.php?image=bank6yv2.jpg
New guild control options.
Each rank can be given/denied permission to manage the bank, Repair from the bank, and see the logs.
Each rank can also have different permisssions set for each of the 6 tabs in the bank. Not displayed in the screenshot next to the text "Guild Bank Tab:" will appear a tab-button for each bank-tab the guild has purchased, and each tab can have different permissions for each rank, i.e. wether or not members of that rank can see that tab, and wether or not they can deposit to it, as well as how many stacks of items they can withdraw from that tab per day.
Obviously since guild banks aren't active yet and because its PTR any of this is liable to change, but this is how it works as far as i can tell at the moment.