Ok having investigated this it appears that the bad boy is: nvidia-common
On my system with GT 240 graphics card the update to this is what causes the panel to dissapear
It may be the same with other systems using the nvidia GT 2xx series, not having any others cards of that series I cannot compare. however I already knew there was a problem with my card as nouveau doesn't seem to recognise it on initial load of live disc or install (which is why 'nomodeset' has to be used). Anyone out there in Mint development land listening???
So what I did to fix this was; do a clean Mint 13 OS install (no problem with data loss as I have /home in a separate partition)
update the nvidia graphics driver, from the Hardware popup
then install updates packages as follows
from the first screen the update manager opens, the Level 1 mintupdate (currently 4.3.9)
Then from the next screen of updates, All Level 1 and 2 updates
and in Level 3 everything Except 'nvidia-common', its about #39 down the list of Level 3 update packages (there were 509 of them)
Answer 'yes' to replacing the config files, as this is a clean install and they have nothing to do with the panel vanishing.
Then reboot and the panel should still be there... This is not really a fix more a work-around which worked for me and my hardware
... Note however, there will be still an outstanding nvidia-common update package in the update manager (I think one has to live with it, perhaps it'll get fixed, I don't think it's Mints fault though)
Hope this helps someone else ... if so put a post up and let us know what you did (also, if you did something else)