In my case, I have never paid for a single app. The last thing I bought on iTunes was a CD a year ago.
I have a few free games and apps on my iPhone, mostly 'top free' and nothing obscure.
The night before I got the unauthorized purchases, I ran the App Store app to update Paper Pilot (free). The iPhone 4 is my first iPhone, so it was odd seeing the App Store ask for my password twice (maybe three times) even though I'm sure I typed my password perfectly. I don't know if repeated logins are common or not. But this was the only thing that stood out in my mind that night.