I have this same problem in 11.04 Ubuntu using gnome with the xfce-terminal. I also have a number of other related screen problems when xfce-terminal is on the screen.
1) Resizing the terminal draws a black box on the top of the screen.
2) Changing themes causes xfce-terminal to redraw and create a black box at the top of the screen and/or screen garbage.
3) gnome-panel, when not in autohide, goes black when xfce-terminal is maximized.
All of these are related to redrawing the xfce-terminal in 11.04.
I'm also using the Nvidia driver.
I think this is a xfce-terminal bug not a panel bug.
I have this same problem in 11.04 Ubuntu using gnome with the xfce-terminal. I also have a number of other related screen problems when xfce-terminal is on the screen.
1) Resizing the terminal draws a black box on the top of the screen.
2) Changing themes causes xfce-terminal to redraw and create a black box at the top of the screen and/or screen garbage.
3) gnome-panel, when not in autohide, goes black when xfce-terminal is maximized.
All of these are related to redrawing the xfce-terminal in 11.04.
I'm also using the Nvidia driver.
I think this is a xfce-terminal bug not a panel bug.