Just a small post. Vicente and I have finished commenting the code of our small XNA Particle Engine we built for the UPSAM speech we gave about XNA and the content pipeline.
The code is not "production quality" and some minor things should be fixed and/or refactored (particle engine drawing inside a game component, alphablending may be broken because of the last-minute changes to pigment/colorize particles...), but it's very easy to understand.
You can find the (updated) zip file containing the slides (spanish & english) and the source code here.