Several users have been complaining that global menu bar is disappeared after setting up dual monitor on Ubuntu 11.04. It happened to me as well. Finally, I found an alternative setting which is not exactly what I wanted, but, it gives me minimum functionality that I can accept.
Run 'NVIDIA X Server Settings' and make it look like these setup, then restart gdm.
With this setting, you will get these features,
(1) dual monitors
(2) move windows across dual monitors
(3) global menu bar on each monitor*
I don't like (3). What I wanted is having dual monitor with global menu bar on primary monitor only. But, I couldn't find a solution for that.
After using this setup several days, I found out that having global menu bar on secondary monitor is actually very useful or must have feature. Global menu bar becomes application menu bar once application is launched on secondary monitor. So application menu bar resides close to application itself. Otherwise, you will end up with application menu bar on primary monitor and application on secondary monitor.
Let me know if you have better solution about this.