I also use Kanban in combination with GTD but in a slightly different manner. I use a card for "a project" (by the GTD-definition) to limit the number of open loops caused by starting too many multi-step projects. I then have subtasks on each card for individual next actions and tag the card with context-tags (as "call", "mail" etc if applicable). I have found AgileZen to be a really good tool for this workflow and it is free for personal use.