In response to Nicholas' waxing on about the history of science. I can't resist that:
Given Google’s ambitious goals to organize all the worlds information, all of it!, I’m sure much more of your correspondence will be hanging around than you’d like for years to come. One crazy futuristic benefit of really saving everything may be a kind of immortality. And I don’t mean the kind of immortality where people talk about what a great/horrible person you were. I mean immortality in the sense that there is a talking, thinking copy of you that thinks it is you (maybe it is?). In order to overcome the ocean of data about someone it may become easier to build a simulation of them using all their blog,facebook,twitter,etc. info. Then we could just talk to the person and find out what they thought.
I'm not saying we're there yet by any means. And I'm an AI researcher, I should know, we are soooo not there yet. But give it a couple decades and we'll be close. Lack of data and computing power are two major limiting factors at the moment. The other factor is inadequately complex models, or perhaps the wrong models. That's what we're working on. But as other's have pointed out, if computing power continues to increase at its current exponential rate (which is not certain) then we won't need to worry about modelling as much as copying. Copying the brain. And all that social networking data is tantalizing if you are looking for some background knowledge to fill your brain with. And once the real person is dead there will be no privacy issues (though I could be wrong about that, "I'm a mad scientist Jim, not a lawyer!")
So you may want to think about that when leaving all your public info out there?
(Interesting question for a later blog...should people have a legal will to deal with their internet persona? Would you want your internet persona laid to rest as well? Or would you donate your social networking brain to science and possibly live again in some form?)