How did the US Airways plane land in the Hudson River without breaking up? I would have expected at least some break up of the plane.

It did not smack into the water all at once. It was landed gently, tail-belly first at a forgiving angle, allowing friction with the water to slow it down before the engines and wings hit. It was rightfully cited as a feat of a skilled pilot who remained calm under pressure.
http://www.ritholtz.com/blog/2009/01/actual-footage-us-airways-flight-1549-landing-in-hudson/

Another thing that had a lot to do with it is the temperature of the water. This happened in January. Colder water is denser and has a different viscosity than warm water. This allowed the plane to skim on the surface easier than sinking deeper into it. A couple of months later and the results would not have been so fortunate.

One of the engines, which hit the water first, was ripped off but you couldn't see it because it was under water. However, a plane can be belly landed on concrete by a skilled pilot without breaking up, so there it no reason it should break up on the slightly softer water.

Yes, it was quite exceptional, even if "the left engine broke away on contact with the river".

"The FAA does not require commercial pilots to train to ditch, regulating instead the distance a plane can stray from an airfield.

Nevertheless, all airliners are equipped with flotation devices in case of water landings. According to FAA regulations, aircraft that travel no farther than 50 nautical miles (93 km) from shore are only required to be equipped with flotation seat cushions. Aircraft that travel no farther than 162 nautical miles (300 km) from shore are required to be equipped with life vests for all passengers. If an aircraft travels farther than 162 nautical miles (300 km) from shore it must be equipped with life vests for all passengers, and life rafts/raft evacuation slides.

While there have been several 'successful' (survivable) water landings by narrow-body and propeller-driven airliners, few commercial jets have ever touched down 'perfectly' on water. There has been a good deal of popular controversy over the efficiency of life vests and rafts. For example, Ralph Nader's Aviation Consumer Action Project had been quoted as saying that a wide body jet would “shatter like a raw egg dropped on pavement, killing most if not all passengers on impact, even in calm seas with well-trained pilots and good landing trajectories."

Also, in December 2002, The Economist had quoted an expert as claiming that "No large airliner has ever made an emergency landing on water" in an article that goes on to charge, "So the life jackets ... have little purpose other than to make passengers feel better." This idea was repeated in The Economist in September 2006 in an article which reported that "in the history of aviation the number of wide-bodied aircraft that have made successful landings on water is zero."

Of note is the January 15, 2009, ditching of US Airways Flight 1549, an Airbus A320 narrow-body jet, which successfully landed in the North River section of the Hudson River mid-river between Manhattan in New York City and Weehawken in New Jersey. All on board survived, showing that inflatable slide-rafts and life jackets can sometimes serve their purposes, although photographs from the incident show that very few passengers were wearing life jackets. After take-off from La Guardia, initial reports cite dual engine failure due to bird strikes at a low altitude. Pilot Chesley B. "Sully" Sullenberger was able to cross the Bronx in a slow turn to the south-west, pass over the George Washington Bridge and ditch the plane in the Hudson River. The left engine broke away on contact with the river. All 155 passengers and crew survived with only one major injury and 77 minor injuries, in part because the plane came to a halt adjacent to the passenger ferry route between NYC and New Jersey."

Some other occurrences:
"On 16 January 2002, Garuda Indonesia Flight 421 (a Boeing 737) successfully ditched into the Bengawan Solo River near Yogyakarta, Java Island after experiencing a twin engine flameout during heavy precipitation and hail. The pilots tried to restart the engines several times before making the decision to ditch the aircraft. Of the 60 occupants, one flight attendant was killed. The survival rate was 98%. Photographs taken shortly after evacuation show that the plane came to rest in knee-deep water."

"On 21 August 1963, an Aeroflot Tupolev Tu-124 ditched into the Neva River in Leningrad after running out of fuel. The aircraft floated and was towed to shore by a tugboat which it had nearly hit as it came down on the water. The tug rushed to the floating aircraft and pulled it with its passengers near to the shore where the passengers disembarked onto the tug; all 52 on board escaped without injuries. The survival rate was 100%."
Source and further information:
http://en.wikipedia.org/wiki/Ditching

Further information:
http://en.wikipedia.org/wiki/US_Airways_Flight_1549