Or you guys can just do it the 'proper' way....
by opening up GCONF editor and going to:
/apps/panel/profiles/default/objects/object_? where "?" is a number that coresponds to your main menu. Look at each one until you see:
"tooltip" for the Name catagory and "Main Menu" for the Value. Put a check mark in the box for "use_custom_icon". Now double left click on "custom_icon". For the "Key Value" enter the FULL path to your new icon