The rules haven't changed - they have just been misunderstood. I was curious after reading your question, so I looked it up. Diana wasn't of royal lineage, she was of aristocratic lineage, but she was actually a commoner. As was the Queen's mother. They became royal when they married royalty. Some countries have had rules about this, though most of them have done away with those rules. But, for another example, a recent King of Jordan (I believe it's the current King's dad) actually married an American, who then gave up her citizenship in the US and was Queen Noor of Jordan. Also, look at Grace Kelly - she was an American who married the Prince of Monaco. (Of course, both American women being commoners :-) ).
However, they do have an issue with the person marrying into the Royal family being divorced and being in line for the throne (not just Camilla, but also during WWII era, too...)
hope this helps!
at 11:07 AM on Apr. 30, 2011