The Leahy-Smith Patent Reform Act, otherwise known as the America Invents Act (AIA), has become law following President Obama's signature.
The text of the act, H.R. 1249, is hard going and the USPTO's America Invents Act: Effective Dates document is a useful listing of the key provisions as well as giving the dates the legislation becomes active.
Perhaps most of interest to those in the rest of the world is that the USA will, for patent applications published on or after the 16 March 2013, become a first-to-file country. This means that in case of conflict between patent applications for the same subject matter, it will be the earlier filing rather than invention date that is vital. The filing date could be in another country under the Paris Convention, where the applicant wanted US protection. Now all countries will use first-to-file, although Brad Pedersen and Justin Woo argue in some detail that it's not quite the same regime.
It has been said that this change will hurt small companies and private inventors, and will encourage quick, perhaps premature filings to avoid possible disputes. It could be argued, though, that the cost (and uncertainty) of the old "interference" actions to determine the date of novelty for the parties involved will vanish, as will the associated costs for the USPTO.
Other provisions concern official fees, where, broadly speaking, small entities and those filing electronically benefit, while there is a 15% rise in most fees, and those who want quick processing can pay a $4500 surcharge. President Obama had pointed out that there was a backlog of 700,000 patent applications waiting to be "opened", and for many getting priority would be worth it. Others would say that it is unfair that money gives you an advantage.
There are other provisions to do with, for example, challenging a granted patent within one year. The USPTO itself says "The new law will afford more certainty for patent applicants and owners, and provide the USPTO the resources needed to operate efficiently and issue high-quality patents."