Actually, I think I've deleted and redownloaded an app that I got for free and is now paid, and I didn't get charged for it. I say "I think" because I was deleting and redownloading several apps at once to try to fix the conflicting updating problem between the Touch and iTunes, so I'm not really sure what I did with regards to each individual app, but I do know that my current solution to the update problem involves deleting and redownloading the updated apps rather than trying to update them, and I also know that some of those apps I've been "updating" in this manner are no longer free. So I'm pretty sure, though not 100% certain, that you can delete a formerly free app and redownload it again w/o being charged.