It's possible via a slightly non-standard method to have offline web apps on the Maemo platform. Tear is a webkit based browser, and when using a recent webkit (more info here) you have access to the local storage facilities as specified in HTML5. The problem is that full AppCache functionality doesn't exist/work yet..
However, you can host your apps locally on your maemo device, and by setting "document.domain = your.domain" in javascript, you can use the security context of the remote domain when making requests.. When you're offline, you just return information from the local data stores, but when you're online, you can get new information easily. Neat, eh?
Still, I'm looking forward to AppCache support so I don't have to save pages locally.