Most netbook users are travelers and most of them stay connected to inernet. Some people have blackberry connected to their corporate networks for emails. But while traveling some may not have "data" access from their mobile provider so may face difficulties in receiving or sending emails.
This app should run without booting the OS. Just like normal mobile devices Mouse support and graphics support can be avoided. This can make sure that the app can start without the help of an OS (or a minimal OS kernel).
App should have basic functionalities like checking emails and replying to them. So in short only keyboard, network, monitor(mono chrome) functionality is all required.
This way netbook can be more useful while traveling especially in airports. No need to wait for the OS to boot unwanted drivers and software.
In addition if required Twitter, Facebook and other social networking apps can be added.