That being said, Anna Forss blogged today about how estimates do or don't fit into Kanban. An excerpt:
Jump to her full post to see how she answers this question.
The main reason for estimating tasks is to be able to decide on an appropriate work load during the sprint. During the sprint planning meeting, the team estimate how big tasks are and when the work load (estimated task sizes) matches the available resources, the team commits to the sprint backlog.
So, how about kanban?
Here's my comment on the topic, even if it is a little open-ended:
This also make the concept of MMF (minimal marketable feature) an important goal when writing stories!