I was a little surprised to see this.
When Prince William and Kate Middleton tie the knot in April, our President and Michelle are not invited. (At least, not at this time - these things have a way of changing.) Prince William wants to make sure that the little people are included at the wedding. He is also actively involved with 21 different charities. 21? He wants the charitable volunteers and workers to be invited, too.
His justification for not inviting the President is that his wedding is not a state event. That is to say, he is not heir apparent to the throne. (Does Kate know this?) Some heads of state, however, will be invited.
Is this a big deal? Maybe it is, maybe not. I should point out that when Charles and Diana married, the Reagans were invited. Nancy attended alone as the President was recovering from an assassination attempt.
I say that the Obamas should be invited. No special reason, except that as the grandson of the Queen, you should probably make some concessions. It's a small price to pay for being a royal and never having to get a real job.