But even if you rebind Q you can still quit the game by accidentally pressing both at the same time.
This appears to be because Command Q is bound in the menu item for "Quit Diablo III"
I think i've been able to figure out a workaround until it gets fixed.
** Work around starts from below **
In OS X Lion you can remap the key combos of programs. If it's in a menu I understand it can be remapped. I'm not totally sure if this feature is available in the other versions of OS X.
You get to it via System Preferences -> Keyboard -> Keyboard shortcuts (Tab)
Click on Application Shortcuts.
Click the Plus Icon at bottom right
In the sheet that pops up :
For Application scroll down the List to "Other..."
Navigate to Diablo 3 folder. Select the Diablo 3 app.
Menu Title is "Quit Diablo III"
You must get the menu title correct and exact for it to work. Note those are capital I's at the end there.
For Keyboard shortcut - Assign something like and complicated that you know you'll never use and Blizzard would never assign to anything. Like Shift Control Option Command Clear.
If you've done it correctly you'll see a "Quit Diablo III" under "Diablo III" with a string of symbols representing the key combo you've chosen. If you've done it incorrectly then it will still have the Command - Q key combo showing.
Now launch Diablo III and when it loads go into Windowed mode via Command M. Or via Options -> Video -> Display
You should now see that in the File Menu "Quit Diablo III" is no longer Command - Q. It should now be the weird combo you assigned above (you know the one that you'll never use anywhere on your mac in a 1000 years :P)
Or rather it should be :P
Pressing command and Q together should now not quit you in game.
Apologies if this is a little unclear. I'm a bit tired and about to pass out. I'll see about doing another editing round to make this a little bit clearer and possibly adding some photos and uploading it somewhere i can have pretty photos and stuffs