PBS is classing up the iTunes Store with a selection [iTS link] of children's and primetime programming. Available for purchase are episodes of Arthur, Cyberchase, FETCH! With Ruff Ruffman, Antiques Roadshow, Nova, Now, and Scientific American Frontiers (which is my favorite of the lot, however, Alan Alda is no Woodie Flowers).
The standard iTunes Store pricing of $1.99 per episodes applies to all the PBS programming.
Update: As Russ points out, Nova episodes cost $7.99. I didn't notice that earlier. That seems a little high to me, but Nova is a fine show.