Update: These messages are part of Apple's security system for preventing people from logging into other iTunes accounts and re-downloading applications that their friends have purchased. You will not have to pay for re-downloading apps that you've purchased.
A few iPhone and iPod Touch users have received a cryptic message on their devices while trying to re-download certain free applications over-the-air (Edge/3G/WiFi). One such application, Google Mobile, is completely free and will not charge a user even after popping up the warning message, but the functionality for re-charging seems to be in place.
What remains to be seen, however, is if Apple will ever actually do this or leave it up to the application developer. Maybe it's pressure put on by AT&T to cut down on 3G usage wherever they can?
Either way, this is just a hint that something like this could happen, but it isn't something to get worked up about at this time. In that case, let's just get worked up again about AT&T and how terrible they are being with the iPhone. I'm generally of the opinion that all cell providers in the US lag behind carriers in Japan and Europe in speed and features, but AT&T has never been bad about locking down phone features. I hope they can deploy that 4G network fast enough for Apple before Verizon snatches the iPhone 4G out from under AT&T.