Topsy, a social search engine, states they've managed to index ever tweet ever made...
...and more to the point, why? I'd suggest that MOST, the vast majority of Tweets, or largely useless. last night I tweeted obscure '80's references while blathering about what music I was listening to. Really. Truely. Irrelevant. ...and yet, in that se aof noise we all create, there remains some nuggets of value.
Topsy - you want to impress me? Sort that stuff, dump the garbage and show me the real gems. ;)