Found this post from Yglesias much more interesting than Kim thought I should:
The crux of the matter is that “prince” and “princess” aren’t real titles of nobility. A real title goes with an “of Someplace” after it. If you’re the Duke of Something then your wife is also Duchess of Something. “Prince” and “princess,” by contrast, specifically denote that you’re the direct descendent of the Queen. Thus Prince Charles is a “prince” as are his sons, and his brother Prince Andrew and his daughters Princess Beatrix and Princess Eugenie. But Kate’s not descended from The Queen so she’s no princess. William was made Duke of Cambridge in part specifically to address this, since as the wife of the Duke of Cambridge she now has a title befitting her position as an important member of the Royal Family.
But there’s an exception here! The Heir Apparent to the throne isn’t just any old prince, he’s the Prince of Wales. Consequently, his wife (if applicable) is the Princess of Wales. So if the Queen dies, Prince Charles will become King, Will will become Prince of Wales and then Kate will be Princess of Wales. Long story short, it’s hard to become a princess (sorry ladies) since at any given times there’s at most one Prince of Wales and the proliferation of other princesses doesn’t help you.
And even cooler, this handy princess flow chart: