While pure software is difficult to patent in Europe, the UK, China, and many other countries, due to a requirement of a "technical effect," there is no such requirement in the United States. This has been confirmed in a recent decision by the U.S. Supreme Court, Bilski et al. v. Kappos.
According to the Supreme Court, the machine-or-transformation test is a useful and important clue for determining whether some claimed inventions are processes under section 101. But the machine-or-transformation test is not the sole test for deciding whether an invention is a patent-eligible "process."
While many commentators lamented the fact that the Supreme Court declined to outline a clear test for determining when a business method is capable of patent protection, I feel that I can claim almost any software invention and most business methods in such a manner as to comply with this test.
I recommend that companies in the UK, China, Japan, Brazil, and India, and elsewhere routinely file software patent applications and business method patent applications in the USA because it is much easier to obtain allowance of a software patent in the USA than in their own countries. Additionally, of course, the U.S. is a major potential market for software and apps.
The USA has no blanket prohibition against software patents. On the contrary, almost any software invention (as long as it is novel and non-obvious) is capable of being patented in the U.S. This includes improvements in user interfaces, novel algorithms, novel applications, and even modifications of existing software.