Running a Feisty Fawn too, and I don't have that problem; are you sure you have upgraded the system so it's not beta anymore? Are you using Beryl (desktop effects) or something else, or just out-of-the-box Feisty?
I had the panels-disappearing problem with Gnome 2.16 (Feisty has 2.18) when running programs over ssh (X11 forwarding) if they crashed. How do you actually produce this thing, just by pressing ctrl+alt+arrow_right/arrow_left or what?
Maybe try to see if there are updates available, or if you're using something non-stock stuff (desktop effects, Beryl, other things that weren't enabled by default) try disabling them one by one and see if that helps.