I believe the current standard is 21 weeks. I seem to remember a primi that lived after 21 weeks a year or so ago.
Wikipedia to the rescue -
http://en.wikipedia.org/wiki/Premature_Babies
The have a nice table in there, as well as references. 24 weeks looks to be the current "standard" but children have lived after being born as early as 21 weeks.
There's an interesting side discussion on this - the viability point will no doubt continue to move backward as medical science improves. At some point the viability point will no doubt be the point of conception. Hopfully by then women won't need abortions for any reason, but it would be interesting to step into the future and see how this all plays out by then.