Our son has been sitting up for a couple of months now. He's trying to walk, but he's trying to skip crawling. One problem I see with the lack of willingness to crawl is that he's not learning to go from sitting to tummy position.
The problem is that he's not learning to fall gracefully, to use his hands to stop his momentum and to know his boundaries.
In order to combat his lack of total balance control, we always put a pillow behind him or put his back to a couch or wall - something that won't allow him to fall back or something that will break his fall.
Doing that, I believe, has caused it's own problems or hindered his development in some ways. When we're in bed, I sometimes horse around with him and push him from sitting onto his back and he thinks it's fun. What he can't decipher, though, is when he's sitting on a bed versus when he's on a hardwood floor. He doesn't know that he can't just lay back full force any time he wants to, or he'll knock his head.
He's done so a couple of times. The latest was last night, when he was sitting against the chez lounge. I was making faces at him and he was laughing. He leaned back, but had rotated around a bit and fell back, hitting his head on the wood floor. Another time was in the tub. He leaned forward to grab a toy, slipped, and hit his forehead in the side of the tub.
He cried a bit, but not too bad. So now I'm trying to think of a way to keep him from cracking his head open, while making sure he knows that he can't just fall back and it's ok. First, I have stopped pushing him backwards on the bed. Next, I'm going to put a folded blanket or towel behind him so that, if he falls, he'll still land with a thud and it won't feel good, but not with the hardness of the floor itself. Finally, I'm going to get a shower mat so that it's not so slippery.
On the good side, he's conditioning his head to take a beating. Once he starts playing sports, he'll need the toughness.