Any "classical" (as it will be called once the technology is in wider use) problem which is solved by "classical" code can be solved using some sort of quantum processor by transforming the problem. For example, to do a database search, instead of using an index-based search/binary search, or a linear search for an unsorted database, you can use Grover's algorithm. Also, to take a step back from the previous poster's mention of BQP problems, problems with a classical "solution" that runs in NP-time can be sped up considerably by Grover's algorithm (a speedup in the time to search through every possible solution). RSA cryptography is also made much more insecure by the advent of Shor's algorithm, since it makes factorising large numbers into their prime factors (the hinge upon which RSA sits) solvable in logarithmic time.
EDIT: Shor's algorithm actually runs in O((log N)^3), which is polynomial-over-logarithmic time.
The conclusion of this sort of thing is that pre-existing programming languages like C will not be able to be used on a quantum computer due to the nature of quantum algorithms (applying certain functions to quantum states), unless someone invents a way to map quantum gates and so forth to logical gates (EDIT: This has apparantly been mostly addressed here), in which case about all we get is a very very fast logical processor when using languages like C.
PS: I'm sure there'll be OpenGL bindings for quantum computing eventually :P