First, a State Dinner is supposed to be reserved only for Heads of State - hence the name. Mr. Cameron is not a Head of State - Queen Elizabeth is. But that is, I suppose, a technicality. The real corruption lies in others who were invited.
It seems Obama invited 41 of his top bundlers, people who raised over $8 million for his campaign. And this raises an ethics question - can a person buy a seat at a State Dinner by raising campaign funds for the President? And if so, does that not constitute a campaign activity? If so, it is ILLEGAL for taxpayers to pay for said dinner. And it was a VERY hefty tab, with over 350 people in attendance, and served a wine so expensive the name and vintage was not disclosed.
Obama has often thwarted the law by campaigning on the taxpayer's dime, which is unlawful. And this particular episode smacks us right in the face.