WinXP, Blender 2.4 RC1, Ati 9700pro, AMD XP 3000+
I'm using Blender with dual monitor support (one graphic card with analogue and dvi connector):
C:\Programme\blender\blender.exe -p 0 80 2550 1220

Popup menus leaves allwas artifacts on the screen or create some where the shadow of the menus are.
But when i leave an area and passing the the border where i can change the size of the area (or split the window etc) the area will be redrawn and the artifacts disappear.
But this didn't happend when the menus just disappear.

Another interesting fact is when i maximize the window, so that Blender uses just one monitor, then i didn't get problems with the menus.

I attaches a pic.

PS: The 3D view it self has just the size of one monitor.