If I whisper the most beautiful poem over the telephone to my lover today, am I going to complain about the phone company not archiving that call properly when I want to revisit it 20 years from now? You go into Twitter knowing it's about the ebb and flow of current conversation, not about what was said when it first came out. If Twitter wants to eventually offer that, that's cool, but that's not the case right now, and you shouldn't expect it any more than you should expect 7-11 to keep their security footage of you for your perusal indefinitely into the future.

edit: from Twitter's Terms of Use (http://twitter.com/tos): "This license is you authorizing us to make your Tweets available to the rest of the world and to let others do the same. But what’s yours is yours – you own your content."

You own your content. You are responsible for it.