Although the agile project team knows when they will finish the next development iteration, they do not know exactly what they will deliver.

By having the courage and wisdom to admit that the result is kind of vague and unknowable in advance, good planning practices dictate a more straight-forward process of setting and revising goals that more importantly lead to a longer-term objective.

Embrace change, be flexible and go for it with everything you've got.

For an entertaining and well-written book covering this concept (and from which I paraphrased the words of wisdom above), please have a look at Agile Estimating and Planning by Mike Cohen.