Where it has always been:
Shift + 2.
This goes for all Apple keyboards and not just the Bluetooth keyboard.
If you open the Language and Text preferences there's an option for the "Keyboard and Character Viewer" tick this option to gain easy access to the viewer via the menubar:
You can use the Keyboard Viewer to easily find characters:
In the above instance Shift was pressed to display the characters on each key. You can press any modifier key or combination of modifier keys to see the different special characters or symbols you can type.