For an app developed as a side project by a team in Australia, Google Tasks has done pretty well for itself. One option it has lacked is import and export features, but Google Tasks Porter fills that gap.
As well as letting you export data in iCal, Outlook and Remember The Milk formats and import data from iCal or Outlook, Tasks Porter can take “snapshots” that show the state of your task list at a particular point in time. That could be helpful if you want to track how well you’re working through your list. Tasks Porter is an open source project and free to use in any browser; you need to authorise it to access your Google account.