Apple will NOT let you re-download those songs. Just like if you buy a CD from Wal-Mart and lose it, Wal-Mart is going to replace the CD for you. Like other people have said, you are responsible for backing up your purchased songs, and the iPod, it NOT a backup. Many people think it is, but the iPod is just a music jukebox, not a backup source. Like sdreed25 said, there are utilities that you can use to extract the music off your iPod. Do a search under http://www.versiontracker.com and you'll get a utility that will work.
I use a program called El iPodo to retrieve songs from my iPod. It comes in handy to pull files from the pod to be used for my other Macintosh computers currently not networked. It works well, but the interface isn't the best out there. If you need to use it, google "El iPodo".