EAGLE PCB software needs to use the <alt> key to switch to alternate grid spacings but under Ubuntu, the <alt> key is assigned by default to be the window picker. In Ubuntu 10.xx you could change this assignment with System->Preferences->Windows. Under Ubuntu 11.10, however, you will need to install gconf-editor and launch it and then navigate to Apps>metacity>general and change mouse_button_modifier to <Super>. This will allow you to use the <alt> key to change grid spacing, etc and use the <windows> key for os related functions.