I've been playing my DS for about... umm... a good few hours (*blushes now that I've realised I've been playing for-SSHHHH!*) today, and my hands feel absolutely fine. ^^;

Maybe it's the way you're holding it?

Hahaha. It's the PSP that hurts me - I get a cramp along the side of my hand after about twenty minutes play time with any of my games.