let me rephrase that :)
pyjamas-desktop can be used for "online" apps just as much as it can for "offline" ones. in fact, you _still_ need a web server, even if it's running on localhost, because applications that conform to the pyjamas API will _still_ have to use XMLHttpRequest to interact with the rest of the world.
that interaction is done behind a neat abstraction module, HTTPRequest.py, but it's still necessary.
so - offline / online, there's no difference.
what you _do_ get with pyjamas-desktop is a speed-increase in your application, thanks to no longer being forced to utilise javascript. in cases where your application morphs into something that requires 10,000 on-screen widgets (not inconceivable for complex business apps) the speed-up of direct access to the blindingly-quick WebKit (http://webkit.org) underlying library is absolutely essential.
not even GWT can offer this, because google haven't bothered to port GWT to the desktop: they believe that the "hosting" mode - running from the Eclipse Development Environment - a whopping 150mb download on its own - is perfectly sufficient.