After a few tests I thought it's time for a first tutorial on how to use the new Blender tracker, that is being developed by Sergey Sharybin during the 2011 Google Summer of Code. Currently the tracker is only available in custom development builds of the so-called tomato-branch.
(grab one here: graphicall.org/gsoc)
I am pretty sure that it will still change a lot, but it is already totally usable.
There are endless scenarios where you can use a 2d tracker, with 4 corner pinning being one of the most obvious ones.
But you can also do image stabilizations for shaky footage as well!
Francois Tarlier will most likely publish a tutorial on that.

I also have to give huge credits for the guys who developed libmv, which is used for the actual tracking algorithms. So thank you Keir, jmichot and MatthiasF!!