The latest State of the Climate data from NOAA is in, with some dramatic results. As you can see in the image above, the twelve month period of May 2011 though April 2012 has proved to be the hottest period for the United States, since records began in the late 1800s.
NOAA says that last summer being the second hottest, last winter being the fourth warmest, and last March being the hottest on record all added up to mean that the average temperature for the lower-48 states was 55.7°F. That's 2.8°F above the 20th century average.
On a state-by-state basis, for 22 states the past 12 months saw record breaking warmth, with a further 19 being seeing top-10 warmest temperatures.
So far this year, the January to April period is running 5.4°F above average, with 26 states (all east of the Rockies) seeing a new record warm period, and 17 more states experience top-10 warmth for the period.
NOAA has much more detailed info for this past twelve months, drilling down into precipitation trends (it was drier than usual but not record setting most places), regional variations in warmth and precipitation, and much more. It's definitely worth a read.