MacBook Pro, OSX 10.5.2 (leopard) , Eclipse 3.3.1
GWT Leopard 1.4.61
Finally narrowed it down to a problem with Apple AWT incompatible with SWT and after a lot of googling and some memories from the distant past found this fix
-Djava.awt.headless=true
Show package contents of eclipse.app >> contents >> macos >> eclipse.ini
Just add the previous line to your eclipse.ini
Hope this helps others

