Prince William and Kate Middleton (soon Princess Catherine, I presume) share a strong Moon in Cancer. I quote myself:
When lovers have the same position of the Moon, they feel close and comfortable. Friends often have the same Moonsign. It makes you feel as if you belong together ('We are family').
This Moon might be important for keeping them together, as their Sunsigns don't match and their Mars are conjunct (for a lot of fire, and risk of quarrel). There is a lucky synastry aspect: Venus tredecile (108d) Venus. That means that although their preferences don't match (as the Venus signs don't match) they will find creative ways to settle things because of love. And that is what matters most when you want to get married...Continue for the links and for the chart drawing of Prince William with the transits of the day of birth of Kate Middeton.
More about couples with the same moonsign:
And about love and astrology (matching and not matching), see Astromarkt:http://www.astromarkt.net/index.php?id=115