Usability 
Google now has a mobile web version of their Beta Tasks web application.
On the full web, Google Tasks is integrated into Gmail. The mobile version is a stand alone page. To view it visit gmail.com/tasks (in some regions including Germany it may be necessary to use googlemail.com/tasks.)
There are two versions of mobile tasks. iPhones and Android devices get an enhanced one that lets you add, edit, and delete tasks and task lists, as well as mark tasks as completed. Lesser phones see a stripped down Task interface which is missing the ability to edit or delete tasks although you can view your existing tasks, add new tasks, and mark tasks as completed.
It's too bad that the enhanced version is limited to iPhones and Android. I suspect that other advanced Ajax capable mobile browsers like S60WebKit and Opera Mobile and maybe even Opera Mini could handle the iPhone version but Google is redirecting those browsers to the basic version.