Twitter is absolutely mainstream now. It's quite weird - in the last several weeks almost all my friends and family have signed up. I'm not sure what the tipping point was, but celebrities appear to have a lot to do with it.
5 million users now -- I expect this to balloon exponentially very shortly.
Kevin and Alex from Diggnation were on Late Night with Jimmy Fallon a couple nights ago talking about twitter and doing an experiment. It's seeping into the mainstream very quickly, but it'll die out just as MySpace and Facebook are.