Another page of this site was bookmarked earlier. This one presents a new look at Bloom's Taxonomy, and presents a series of app evaluations sorted according to level. Screencasting has the potential to fit into a number of categories.
These same several steps are enumerated in other articles as well. This site gives a fairly detailed overview detailing how to create top-notch screencasts.
A recent blog post detailing the author's use of Explain Everything in her kindergarten classroom. The basics of good screencasting (which extend into other arenas as well) can be taught beginning at a young age!
Not directly related to screencasting, but an excellent review of seven key ideas in education. Many of these "essentials" can be met through the use of screencasts, whether teacher- or student-created.