I doubt if OS X was entirely hung - probably just Aqua. OS X is made up of two (main) components - Aqua and Darwin. Darwin in the UNIX bit and Aqua is the fancy GUI on top. If Aqua hangs then it will not accept input from keyboard or mouse. However if you have a network (and you have the computer set up to allow this) you will be able to terminal in from another computer, restart Aqua and off you go. I remember in the early days of X windows on other UNIX platforms having similar issues.
Where this is important is when you are using OS X as a server - such as routing/file server. Even though the front end will hang the computer will actually keep going and will work fine for its main use.
R.