You can't redownload things purchased from the iTunes Store. When you purchase something, you are reminded that you are responsible for backing up your iTunes purchases in case something goes wrong.
WHAT? thats crazy - i bought 2 games a few years ago, and deleted em by mistake - with sony psn store, you can download up to 5 times, surely they should do the same with purchased items.?
You can re-download apps as many times as you want. If you paid for it once you will never be charged again. Music, TV shows and movies are a different story. I think SilentPanda has it right though. If you contact Apple they will provide you with a link up to one time. I have a friend that had to do this.