One of the extremely useful keyboard shortcuts for Eclipse is Shift+Control+U which finds all occurrences of a selected identifier in the current file. Unfortunately  this doesn't work under Linux with Gnome because Gnome uses this shortcut for composing Unicode characters. You can check it by typing Shift+Control+U followed by 123 and a space into your browser's address bar: first it will render an underlined u and then a strange g-like character.

The conclusion is that you cannot use the shortcut Shift+Control+U and perhaps also a few others (I experienced troubles with Shift+Ctrl+A) in Eclipse under Gnome. You can the shortcut in preferences accessible e.g. via pressing twice Shift+Ctrl+L. For instance Shift+Ctrl+F1 is OK.

Environment: Eclipse 3.4/3.5, Ubuntu 9.04 with Gnome 2.26.1.