In the screen grab above, I've pressed Alt-Tab and reached the terminal window, then pressed Alt-` and am cycling between the two terminals. In practice you keep your thumb on the Alt and the pinky switches between the tab and ` (back-tick is above the tab on UK keyboard, it might be ~ on US keyboard, my memory fails).
GS 3.6 as on Gnome Ubuntu Remix, no extensions
Alt-Tab switches between applications, irrespective of how many windows a given application has open.
Alt-' switches between windows of the currently selected application.
https://dl.dropbox.com/u/8403291/gs-alt-tab.jpg
In the screen grab above, I've pressed Alt-Tab and reached the terminal window, then pressed Alt-` and am cycling between the two terminals. In practice you keep your thumb on the Alt and the pinky switches between the tab and ` (back-tick is above the tab on UK keyboard, it might be ~ on US keyboard, my memory fails).