Harry Potter canon question

In the movie of Sorcerer's Stone (I don't remember whether it's the same in the book or not), on the train to Hogwarts, Hermione says (in a shamefully snotty rebuke to poor Ron) that she's tried a few simple spells at home, and they've all worked. But what about the Underage Magic Decree?

Not that it matters at all -- it's just bugged me off and on. So what's the scoop there?

(Also, why am I up before 9 on a freaking Sunday?)