I also think there was one recently around 21 or 22 weeks that survived...I seem to remember because my dad sent me an e-mail about it (and it just so happened that I was right around the same number of weeks, give or take a few). But maybe this was an old one and he just refound the article to send me! Either way, I agree with everyone else that the earliest was around 21 or 22 weeks...but every week that little baby stays in there gives her a better chance.
I've also thought about this many times (I've had a few times to worry throughout my p/g)...now that I'm 35.3 I'm still worried...and getting excited, so I have to keep reminding myself to hang out and wait until 38 weeks is best! (Then I'll start hoping for the birth!)