Matt Haughey has written a TrackBack implementation for MetaFilter. You can read more about it here.

For now, it's more an experiment to see how TrackBack can serve to supplement the existing discussion atmosphere. It is, of course, very exciting because of MeFi's large user base; the more people using TrackBack (or, at least seeing it being used), the more likely new uses will be discovered. And of course, the greater chances of other developers to implement TrackBack into their own tools.