The America Invents Act (AIA), signed on September 16, 2011, brought upon the patent law field, the most comprehensive change in statutory patent law in the past sixty years.
The most controversial of these changes is the switch from a First-To-Invent to a First-To-File priority regime. Prior to the AIA, the inventor who come up with a concept first could get the right to obtain a patent regardless of when he filed the patent application. (subject to certain requirements) After the AIA, the inventor who filed the patent application first, regardless of who came up with the concept first, could get the right to obtain a patent.
I recently gave a presentation to the University of Arizona College of Engineering Industrial Board, describing these changes and laid out various strategies that inventors could use to manage uncertainty brought upon by these statutory law changes.
Anytime the law changes, especially when it shifts dramatically, it brings about legal uncertainty. Every new word, sentence, and clause will be parsed and litigated for the next thirty years. New regulatory procedures were created by the USPTO to supplement these statutory changes but there is no real track record as how it will play out since they have only recently been implemented. Here is a link to this Presentation