In recent 2.11.x metacity versions (ubuntu, but tested also with CVS), my gnome
panel started disappearing, at some point in time, when switching to another
desktop.  To reproduce the problem:
1. select any given desktop;
2. open a drawer in the panel and close it again;
3. now switch to another desktop -> panel disappears!
4. switch back to the original desktop -> panel reappears!

From then on, the panel becomes pinned to that single desktop.

I have run xprop on the panel before and after the problem occurs:

----------------
gjc@emperor:metacity$ xprop >& /tmp/panel-props.before
gjc@emperor:metacity$ xprop >& /tmp/panel-props.after
gjc@emperor:metacity$ cd /tmp/
gjc@emperor:tmp$ diff panel-props.before panel-props.after
1c1
< _NET_WM_USER_TIME(CARDINAL) = 4398752
---
> _NET_WM_USER_TIME(CARDINAL) = 4443064
7c7
< _NET_WM_DESKTOP(CARDINAL) = 4294967295
---
> _NET_WM_DESKTOP(CARDINAL) = 0
gjc@emperor:tmp$
----------------

If I kill metacity, it respawns and everything goes back to normal:
----------------
gjc@emperor:tmp$ killall metacity
gjc@emperor:tmp$ xprop >& /tmp/panel-props.after-kill
gjc@emperor:tmp$ diff panel-props.before panel-props.after-kill
1c1
< _NET_WM_USER_TIME(CARDINAL) = 4398752
---
> _NET_WM_USER_TIME(CARDINAL) = 4493124
gjc@emperor:tmp$
-----------------

I'm not sure whether it's a metacity bug, or somewhere else, but...