Amazing... another thread with the same question.
I have replied to RogerLu a reply with a solution for what you want. There is also a thread asking for that not long ago in the forum, most likely started by RogerLu. Try giving him a shout.
It's not hard at all, the registers are there, you only need to add the right method.
Here's the thread:
http://arduino.cc/forum/index.php/topic,78209.0.html
Again, I have already looked it up for RogerLu, so it may be a nice idea to get that from him.