This may be irrelevant with Gnome 3, Gnome Shell and Unity taking over, but if you have dual monitors and want the Gnome panel (e.g., menu bar) to span multiple monitors, you can follow these instructions:

Here’s a step-by-step way of moving a panel to another screen:

1. Right-click the panel you wish to move and select “Properties”.
2. Uncheck the “Expand” option under the “General” tab.
3. Grab one of the edges of the panel by clicking on the left or right end (top or bottom end for vertical panels).
4. Drag the bar to the desired screen and position.
5. Check the “Expand” option in the “Panel Properties” window and click “Close”.

From Chris Jean’s blog.