i cut terra cotta up with a carbide blade, for a jig saw i got at home depot, held in place on a wood dowel with a hose clamp. it works good and only cost like 2 bucks. they also sell carbide whole saws that quickly cut holes in clay pots but they are kinda pricey.
http://ourworld.cs.com/kraut491/saw.jpg
http://ourworld.cs.com/kraut491/saw.jpg