I noticed that too! I had a billion missed calls from people this morning because the vibration didn't wake me up. I tested it out on my old iPhone 3G, and it seems to me, the vibration is the same for both, it's just a LOT quieter on the iPhone 4.
The 3GS vibrate was much better and the best. Iphone 4, is cut in half of what the 3GS used to be!