iPad 2 vs iPad 3 stylus sensitivity
There is no way to adjust the pressure sensitivity unfortunately. However, there are several pressure sensitive styluses (styli? I've never had to pluralise stylus before) on  the market or at least in development. 

Check out the Jot-Touch.

The Pressure Pen and Ten One's Blue Tiger are still in development.

Wacom's Bamboo Pen isn't pressure sensitive, but it does improve pressure sensitivity.