There appears to be a project on Kickstarter to create an open source pressure sensitive pen. The current design doesn't look like it incorporates a "higher precision" tip like the iPen (http://www.kickstarter.com/profile/ipen) or Jot Pro (http://adonit.net/product/jot-pro/) that have already been mentioned, but it could be promising.

http://www.kickstarter.com/projects/option8/pressurepen-open...