The military and the Energy Department chose a new design that was developed by the Lawrence Livermore National Lab, the other competing design came from the Los Alamos National Lab. These are the two premier nuclear weapons labs in the government and have had their proposals scrutinized over the past year.
This new design is supposed to eventually replace the aging existing arsenal of weapons, they should be safer and more robust which should increase their reliability compared to the current designs which are a maintenance nightmare when they’re expected to last decades (assuming nuclear war doesn’t break out in the interim).
It’s estimated that as the old warheads are replaced with the newer models, the total number of nukes will likely be reduced from 6,000 today to about 2,000 in the year 2012.
If you’re curious about the ‘why’ of the program you should read this 2 pager from the DOE ‘Reliable Replacement Warhead Program‘ (PDF).
Read the USA Today article on the event as well.