According to the US Patent Office, a patent covers any "new and useful process, machine, article of manufacture, or composition of matter".  Now, in terms of software, the only one of these that applies is the "new and useful process".  But how does that apply to software?  Right from the Patent Office's FAQ, there is a line that says, 'The word “process” is defined by law as a process, act or method, and primarily includes industrial or technical processes.'  So even if we take that at face value, we can say that functions and classes could be patented.  But can the functionality of a program be patented?

         The patent office also  allows for Design patents, which covers the ornamental design for an article of manufacture (I'm assuming this could be applied to a user interface of a program).  If it does apply to the UI, then the interface portion of the program could definitely be patented.

        But what about the functionality?  Is a program patentable as a concept?  Nobody has a definitive answer to that question.  The source code of a program (and the program itself for that matter) is covered under copyright protection.  Shouldn't that be enough, or do we demand more protection?  Should I be able to sue because someone else made a program that does the same thing as mine?  Even if they never used my program?