I've said from the moment the iPhone 3G was announced that Apple should sell last years model SIM free from their own website, and that the exclusivity agreement should only count for the current gen handset.
Think of the extra users / cash Apple could claim now by selling the iPhone 3G SIM Free through their sales team, website, and retail store.
Easy money. Why wont they do it??
As for the networks unlocking them once you have finished your contract, it would be nice, but can't see it happening. Sorry.