One thing that I've wished the iPad had from the beginning was the ability to detect different levels of pressure, similar to a Wacom tablet. That would make it much more useful for things like sketching. I've heard from a few people that the hardware supports it, but I've been skeptical of those claims. How could a capacitative touch device detect pressure? But a few people I talked to at WWDC insisted it was possible with the hardware.
Turns out they were right. This is really cool. I don't know anything about the technology - whether they're really using pressure (I tend to doubt it) or just the relative size of the area being touched, but the results in the video look promising. I also hope the final version can be accomplished with only public APIs somehow.