I wouldn't have been surprised if the penalty was given, but there is some doubt about exactly where it hit his arm - if it's on his sleeve, then it's not a penalty. If it hit below his sleeve, then it has to be deliberate for a pen to be given.
I think he was trying to control it with his chest, but the spin of the ball caught him out and it hit his arm, so in this case not deliberate.
Still, what do I know? All of those so-called experts, like Shearer, say it was definitely a pen, so it must have been. Unless he's just a c*nt.