One more particle system tutorial added to Vimeo. This one explains how it is possible to destroy a mesh with an external particle system using the reactor particles and the explode modifier.
Any comments are welcome, especially if there are some factual errors.
ps. HD version didn’t appear on Vimeo page for some reason. So, if you can’t see what is happening there, you can use the downloadable version, which’s link is on bottom of that page.