I had exactly the same problem with my little one.
I called the hospital a few times! after he hit his head, VERY scary.
At the hospital they actually said to make sure you look for the signs suzqld mentioned for 24 hours. Apparently it can take up to 24 hours for the signs to appear when it comes to head injury.
Eventually Dean learned how to fall on his bottbott and now we dont worry anymore.
It seemed forever until he stopped falling on his head. He used to stand on furniture and then fall backwards straight onto his head. His head even bounced a few times off the tiles, he was OK but I wasnt.
Dean - Born 16/06/2003http://www.freewebs.com/deanmorris/