I apologize if this is not handled by xfwm, it's just my guess that it is. I would find it very useful to be able to 'Escape' out of the alt-tab dialog and revert back to whatever application previously had focus. Sometimes you start alt-tabbing, then realize you don't actually want to switch applications, and the current behavior requires you to find the original window in the alt-tab dialog again. I think most users would expect the ability to escape out of it, and I'm pretty sure some other window managers work this way.
Thanks, keep up the good work ;)
Makes perfectly sense, I'm missing it too.
It's now in SVN trunk.
Works perfectly, thanks Olivier!