In the scene where Wolverine stabs Rogue through the chest it shows a shot of her back with the claws sticking out through her shirt. But after Wolverine retracts his claws it shows her back again and there is no shirt covering it...
Thats right, look again jester, its her bare back, and what a nice and slender one it is. Then if you look again, the babe's back begins to heal without any shirt. Also if claws were to come through her back followed by her shirt, it would have made her shirt push out a bit from her back (try poking your shirt from inside towards the outside and you will know what I'm talking about). Who has their shirt glued to their back? This slipup has not been obseverved very well.
His claws are super sharp, so they would go completely through the nightgown. Remember when he sliced a piece of the Statue of Liberty off? Her back his showing because they wanted to make it look like her shirt was ripped, but it wasn't. It was just to show the healing so stop being so picky.