The majority of browser vendors are introducing offline web application functionality within HTML5. It allows an online application to work even if the user loses or disconnects their internet connection. For example, you would be able to compose a message in your webmail client even if you couldn’t find a wi-fi hotspot.How To Create an Offline Web Application

Great post by Craig Buckler at Sitepoint on the principles to get your application working even when the user is not online. As the mobile web increases in importance, and connections not always being fast or even available everywhere it’s nice to have the users still able to use the site, cache their transactions and then synchronize when they’re back online.

There may not be any killer offline applications, but that doesn’t mean there will never be. Unless ubiquitous and reliable mobile broadband shows up first.