Is it possible to set up a repeating task that does not repeat forever; e.g., "every Monday at 9:00 AM until 12/1/2013"?
Unfortunately at this time it's not possible. We are going to consider adding this ability in the future. Feel free to vote on this: http://todoist.com/Vote/showProposal/214/
In the meantime, you can use CTRL+ click on the task to complete it for good, it will not repeat after doing so.