Bug 443763 (CLOSED, UPSTREAM) details a keyboard layout change problem: gnome-keyboard-properties uses "Both Alt keys together change layout" as default if two or more layouts are selected. Recent xkeyboard-config versions (including the ones in F9 and F10) cannot trigger this due to freedesktop.org Bug 4927. The fix for FDO bug 4927 is likely to drop the xkm format which will not happen until at least xserver-1.6. Meanwhile, a different default in gnome-keyboard-properties to switch between layouts would be good as a workaround. Note that this bug also affects the "both shift keys ..." and "both ctrl keys..." options.
I changed it to shift-capslock in 2.24.0.1-5.fc10
It works fine, thanks. Having used it and thought a bit about it: Based on experience from old type writers I would assume that shift-capslock had something to do with modifying capslock to make it more or less persistent and annoying. The Alt keys are usually used for entering symbols from other character sets; a kind of temporary alternative keyboard layout. So IMHO alt-capslock would make a lot of sense. It might however not be worth the effort to change it again, but I would just mention the idea ...