I've got an iMac 11,3
Intel Core i7
2.93 ghz Quadcore
8gb Ram
ATI Radeon HD 5750
Mac OS X 10.6.5
I followed CX Games 9 Alternate How To in the Tips & Tricks Tab.
To get past the 'falling cursor' mouse problem, I played with the video settings under 'Options' in the initial screen that I get when I press 'Play' in Steam. I basically tried different ones until I found the one that exactly matched the resolution on my Mac. I went to the Apple Symbol in the top left corner and clicked on 'About This Mac'. I then clicked on 'More Info' and Graphics/Displays. I then tried to find my iMac Display resolution in the Fallout 3 options. I eventually selected 16:9 widescreen (2560 x 1440).
Fallout 3 now plays in fullscreen with no mouse troubles.
HOWEVER ... when I was '19' (lol) the game froze on me. I suspect there was a cutscene in there. It seems the game plays okay though. Pipboy works fine and I killed some roaches and some random guy.