Unfortunately there's no way to completely turn off mouse acceleration in Mac. All you can do is to turn the tracking speed (or whatever it is called in english) all the way down under mouse settings and then use another application to make the cursor move faster. But even with the tracking speed turned down, there's a little acceleration.
Hopefully Apple will hear the cries of Mac gamers now that Steam is here.
PS. To test mouse acceleration, move your mouse so that the mouse cursor touches the left side of the screen and at the same time the mouse is touching an object (keyboard for example) on your desktop. Then move the mouse rapidly to right, but make sure the cursor doesn't touch the right edge of the screen. Then slowly move the mouse back to left so that it touches the object again and look where the mouse cursor is now:
If the mouse cursor didn't reach the left edge of the screen, there's acceleration.
If the mouse cursor touches the left edge of the screen at the exact same time the mouse touches the object there's no acceleration.
If the mouse cursor touches the left edge of the screen before the mouse touches the object, there's negative acceleration.
Those results may not always be accurate, if for example the mouse isn't capable of tracking the surface accurately during rapid moves.