You can add tasks without a due date in a few different ways:
First, you can simply leave out a date when requesting to Siri:
Remind me to read Jane Goodall's book
Second, if Siri responds by asking, "When would you like me to remind you?" you can say:
Last, you can also add the task in another way:
Add "Read Jane Goodall's book" to my Remember The Milk list.
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