In the default install of Ubuntu Feisty Fawn on an Apple Intel Macbook, the hash key doesn’t work.
This isn’t really a surprise because it barely works in OSX (Alt/Option 3).
To mimic this functionality in Ubuntu go to System -> Preferences -> Keyboard, choose the tab labeled Layout Options.
In the Third Level Choosers list tick the box named: Press Left Alt key to choose 3rd level.
Now you have the # (hash) key by pressing the Alt/Option button and 3 at the same time.
This is pretty damn useful if you like doing include statements in C/C++ or writing comments in scripting languages like Perl, Ruby and PHP.