MBanz - my install is a hybrid. It started out as PCLOS LXDE 2009.4. I was impressed with that and the lightweight DE principle. So I decided to try xfce4 and downloaded from Synaptic the task-xfce package plus the plugin package. Apart from this one blip, it all works very well - fast, efficient, attractive. But like you, I miss the Alt+Tab keystroke. But like I said, if you go to the XFCE website it's clear from that that Alt+Tab is indeed a feature of it. So why mine's not working I don't know. Except perhaps the hybrid is showing.
Three hours later - having written the above, I was a bit concerned at just how 'hybrid' my install was. I then discovered a thread about the XFCE iso, downloaded it, burned it and installed it on a spare machine. I haven't had chance yet to really give it a test, but I will hopefully tomorrow. So my above comments I'd like to hold in abeyance until I've done that. I guess I should have taken a more careful look around before posting.