There are quite a few people out there doing this. It's not directly built into the product, but you can use GoToMyPc, GoToMeeting or some other screen sharing application to watch someone as they work through a card sort on their screen.
Now that Skype has screen sharing built in as a feature, you can do it for free. As a bonus, a lot of people already have Skype installed so there is no setup to perform.
Let us know how you get on, or if you need any more help to get started.