I started wondering if it could be invaded while playing World in Conflict.
The main problems with invading any country are its geography, the population, and the Army.
1) The United States is a massive place, the third or fourth largest in the world, depending on which set of figures you use. This presents a problem as invading country's armies are often stretched too thinly to be effective. Unlike the two larger countries, America is relatively warm, compared to the freezing winters that defeated the French and Germans invading Russia and the Americans in their failed attempt to invade Canada in the war of 1812. The Rocky mountains would be a problem as well, but only if resistance fighters set up there and attacked the surrounding area.
Overall, I believe that with a large enough army, the size of the US would not present too great a problem.
2) The population of the US would be a massive problem. Thanks to the misinterpretation of the US constitution (It actually gives the right to bear arms to 'A well regulated Militia, being necessary to the security of a free State'), many Americans own guns and are very proficient in their use. The Americans in general have a very patriotic attitude, and the rabid patriotism in their media, which is especially noticable on FOX news, is reminiscent of the patriotism in Nazi Germany. This would likely motivate them to fight to defend their homeland. Their use of actual tactics would be debatable, as they might try to copy Rambo, but they would certainly gather into small groups of resistance fighters. Resistance fighters are always a headache for invading troops: The Eldelweiss Pirates in France killed the chief of the Cologne Gestapo in 1944, and many other groups sprang up in Europe during World War II. The American resistance fighters would be far better armed, and would likely join up with the local National Guard units.
In conclusion, the resistance would be a massive problem for an invading force.
3) The US Army is one of the best in the world, and one of the better equipped (This has improved vastly since Vietnam). They also have around a million soldiers, which could be put to great use and their numbers would be bolstered by Resistance fighters. They also know the are they'll be fighting in well, which is a massive benefit in urban combat.
In conclusion, I believe that if America was invaded, the invaders would face a massive struggle against the US Army forces currently in the country, but if they defeated them, they would be in their own Vietnam-esce war, with constant guerilla attacks. What do you think?