wow. quite a statement.
http://www.pcworld.com/article/158480/offline_gmail_is_almost_like_the_real_thing.html
Their screenshot (http://www.pcworld.com/zoom?id=158480&page=1&zoomIdx=1) seems to imply you'll find the Offline functionality in the tab Settings::Labs (http://mail.google.com/mail/#settings/labs).
earlier PC World article here (http://www.pcworld.com/article/158461/gmail_goes_offline.html) -- apparently it's being rolled out first to just a few folks at a time (http://gmailblog.blogspot.com/2009/01/new-in-labs-offline-gmail.html).
Google's teaser video here (http://www.youtube.com/watch?v=cOAZaIaeIrI).