|Image from here.|
Congratulations to Prince William and
Update: Oops, she was not given the title of Princess because she was not born into the royal family, nor did she marry the heir to the throne. Prince Charles is next in line.
Congratulations to Prince William and the Duchess of Cambridge!
Oh my, they don't even sound married here - not very symmetric.
Congratulations to the Duke and Duchess of Cambridge!
But we're used to calling him Prince William, and this sounds like a demotion for him. Buckingham Palace stated "If people wanted to call Catherine 'Princess', they were welcome to do so." Well then:
Congratulations to Prince William and Princess Catherine!
I like the sound of that! Read more about their titles here.