If you own and iPod touch or an iPhone, you might have bought an app, deleted it for whatever reason and then downloaded it again from the App Store. Since Apple keeps a record of your purchases, you were presented with a dialog box saying that you could re-download that app for free.
According to iPodNN, some developers with preview copies of iPhone OS 3.0 have seen a dialog box (pictured at right) prompting them to either re-download a missing app for free via their computer, or simply buy it again via their iPhones. Who knows why this could be, or if it will even make the final version of iPhone OS 3.0 -- one might think it's to save AT&T bandwidth, but why not restrict the option to WiFi?
In any case, we'll find out soon enough.