I came across this one just recently – if you right-mouse click in your code, and navigate to Compare With > Local History, a History panel pops up.
Listed will be the various versions of your file, and all you have to do is double-click on one of the dates within the list to see that and the current version side-by-side.
What is even nicer is Flash Builder will highlight the areas that are different and also show you project lines between the files indicating where the new/old bits were/are.
Very nice.
