Done for the next update (not live yet). With the Inventory, Skill window, Buddy/Party/Guild list, Quest Log, Storage, shop and other tabbed windows in front, Tab moves to the next tab and Shift+Tab goes back. It only affects the window in front and never fires while you're typing in chat. Change Channel has no pages, so it isn't included. Please give it a try once it's out!

Marking this fixed. Also adding: in the Change Channel window, Tab and the arrow keys will move between the channel options. This is coming in the next update along with the rest.