With Mac 'Spaces' (in Leopard) switched on, in Squeak choose 'full screen on' (rather nice as Squeak is genuinely fullscreen with no Apple menu at the top). Then select 'Spaces' and drag Squeak to a different space. This puts Squeak into a window. Click on Squeak in the space box, and it takes you back to using Squeak, except that Squeak is now in a window. Go to the Appearance menu and select 'full screen on' and nothing happens and Squeak stays in the window - however, select 'full screen off' and then 'full screen on' and the Squeak becomes properly full screen again.
The 'full screen on' menu item is not aware that Squeak has been put into a window by the MacOS.
The fix probably will require something like 'full screen on' to do a 'full screen off' first just to ensure that the 'full screen on' will then work properly.