The Queen has requested David Cameron to form a new government. He is thus the prime minister of the United Kingdom. But he presides over a hung Parliament, and so must now form a government with the Liberal Democrats. It's sort of like the Republicans needing to form a workable alliance with Ralph Nader and the Green Party. It won't be pretty.
Obama is calling Cameron in order to reach out to the new British government. The Tories have been rather cool toward America, while the Lib Dems border on outright hostility. So, now that Britain is a bit more adversarial and anti-American, perhaps Obama will finally begin treating them with a bit of respect.