Toodledo had been testing a new development since January 2012. They called it Offline Support and has been fully tested on Mac Desktop (Safari, Chrome, Firefox, Opera) and a lot off aditional platforms.
The new mobile website uses HTML5 technology to cache data in your web browser and work offline when necessary. If you allow it, the mobile website will download all of your tasks and permanently store them in your web browser. When you disconnect from the internet, you can continue to read and edit your information. All of the changes will be saved inside the web browser and it will automatically sync the differences when you come back online.
I’ve been using Toodledo since 2008 and, with no doubt, is the most powerfull task management tool now.