Hi
I need to control a stepper motor that needs 24V 750mA max. for this I bought the Arduino motor shield rev3. Though now I noticed that this should not be used with more than 12V, it says this could damage the board.
I wonder, would it really be that bad? The L298 can run at up to 46V. Looking at the schematics I think it will just fry the LEDs, or?
Do you mean the Motor Shield Version 2, revision 3, or the Version 3?
Perhaps a link to your exact board?
Edit: Forget it, I just found it.
From what I can see, that warning is there because when VIN_CONNECT is bridged, (the default), the full voltage will be applied to the Arduino Vin. (You don't want 24V to the Arduino.)
It says that the CPU board might be damaged. Meaning the Arduino board, and it WILL be damaged.
I think that if you open that connection and power the Arduino separately, you'll probably be OK.
I say probably because it's late at night here, and I might have missed something. Do it at your own risk!
It is this one:
Yep, I found it, as I said in my last post.
This is that warning. Note that it says the default is "closed"
So you need to ensure that you open/cut that link, and power your Arduino from another source.
Oh, this 12V limit refers to the Arduino supply voltage.
I will let you know what happens 
Might take a few more weeks though until I have everything.
Just noticed that the limit on the flyback diodes is 40V. This might be the maximum.
You'll find that link on the back of the board. This is it:-
Edit: And do keep us posted, and let us know if everything goes up in a big puff of smoke.
(That shouldn't happen. )
The info might be useful for others in the future.
24V 750mA does not sound like a stepper motor spec to me.
Please post details of the actual motor - it may not be suitable for a motor shield
and need chopper drive.
Failing details at least the winding resistance...