That's #1 on the "to do" list.
http://www.mysql.com/doc/T/O/TODO_future.html
There is a quick work around described here though:
http://www.mysql.com/doc/U/P/UPDATE.html
What you should consider here is how often you would be performing this functionality.
If it's a "once every so often" process, I wouldn't sweat it, if it's a very common feature that everyone will be using quite often, then you'd be better off with postrgres or another DB that supports atomic updates.