When my session opens, sometimes cpufreq-applet doens't draw itself on the panel (see attachment).
Left/Right button click and changing preferences (which work perfectly per se) doesn't help : at best the white square gets bigger if I choose to display "graphic and text" instead of just "graphic".
Removing the applet and adding an identically configured new one to the panel is a workaround, but not acceptable for a standard user I think.
On my machine this bug appears 50% of time (this is a rough estimation).
Hardware info can be found at http://deep-ocean.net/files/sony/
I'm on up-to-date intrepid, and this behaviour as been seen from alpha 4, when I joined the lucky-alpha-debuggers because my video card was not supported under hardy ;-)
My WM is compiz. If I switch to metacity (System→Preferences→Appearance → No effects), the white square turns into a transparent one : applet is stills here (i can click, change governor, etc), but invisible.
If I switch back to compiz, the applet stays transparent.