On my iPod Touch I’m using Appigo’s Todo application. I had been syncing the data to Toodledo so that I had a backup in case something happened to my itouch. Now Appigo’s Todo app supports Dropbox. The advantage here is that the place that I’m syncing to, dropbox, supports all of the features of the Todo app. This is important for recovery. Toodledo doesn’t support hierarchical todo items. However when syncing with dropbox it’s in Appigo’s own format so all features are supported. And it turns out the native format is an sqlite database, so I can pull all of the items out of it in case Appigo goes away!
2/20/2012 update. I’ve gone back to Toodledo.com for syncing as the Dropbox sync for the todo app was just too slow. The Toodledo sync is faster, but it’s not a smart as it keeps trying to sync when I’m offline and then popping up an error dialog. I’ve contacted Appigo about this, but haven’t seen a fix yet.