From Mac Guides
The SystemUIServer is a background process that controls several aspects of the Mac OS X user interface, and is one of the applications that are constantly running, when viewed with the Activity Monitor.
Specifically, it controls the right hand side of the Menu bar, where the Menu Extras live, as well as handling application launch following CD or DVD insertion, and handling external devices, such as the iPod.
A bad hardware state may cause the SystemUIServer to hang. Often, unplugging the external device will unhang the SystemUIServer.
The SystemUIServer is owned by the logged in user (as opposed to root) and automatically restarts if it crashes or is force quit.