On September 16, 2011, President Obama signed a new act “America Invents Act” that is going to radically change the way Americans file patents. Found here: http://judiciary.house.gov/issues/Patent%20Reform%20PDFS/112hr1249eh.pdf. The Act will become effective 180 days after signing, - just enough time to understand the new system introduced by it.
Although not an expert in this field, to me, a major difference introduced by the Act is the change from the “first to invent” to the “first to file” system, which is more in line with the patent systems in the other countries.
For a detailed review of the Act, check out the article written by attorneys at Sunstein Kann Murphy & Timbers LLP, a Boston-based IP firm, found here: http://www.sunsteinlaw.com/publications-news/news-letters/2011/09/Sunstein_Blau_201109.html.