It’s the ugly limitation of AJAX apps as we know them. If you are in one of the few places left on Earth without Internet access, the pervasive everywhere-app is suddenly non-functional. After all, the logic of the program lives on the Internet, and even with sophisticated AJAX apps, you are still frequently exchanging pieces of data from client->server and displaying the results, nothing revolutionary there.
Now there are those who would suggest that the vestigial offline mode is passe; a relic of the days before it became a MANs world. However, in their quest to take over the desktop application space, Google has decided that offline client storage is a noble endeavor. In their infinite wisdom they have created Google Gears, an easy way to make an AJAX app just as useful when disconnected as it is when connected. So you can edit your Google Document while on an airplane without firing up something expensive like Word. We’ll see where it goes, but my hunch says this is the coolest thing since AJAX/COMET made responsive web apps a reality.