As restrictive as the iTunes music store is, there is a secret policy in place that will allow you to re-download your entire purchase history in case anything happens to you music collection.
In the event this happens, simply email Apple’s iTunes support group and after a few days you will get the response:
In the event that a customers entire music library is lost, the iTunes Music Store does re-grant the purchases history. Please keep in mind that Apple does not offer protection against the loss of purchases, so this is a one-time exception.
I can verify this is true as well.