Finally got back to this project and did a bit of work. I started using the Maestro speed commands to control the rate at which the servos move to their new position. This helped a lot with the smoothness. Also added an abdomen part which holds the battery and voltage regulator. The walking is much better now, and faster than I had hoped . It could actually be sped up further, I am not at the servos' limit yet. I made a quick video to show it doing IK moves and some walking towards the end. Sorry about the miserable webcam quality!
I have also implemented a simple autonomous wandering around behaviour using the sonar. However it's not very reliable; for some reason the sensor is giving me pretty inaccurate ranges, possible because of the way I have it mounted or because it is too close to the floor. I am going to try to get this working better. Also I have an IR sensor to add along with a bunch of whisker switches for objects it doesn't spot. My current goal is to have it wander around the house without getting stuck. After that I can begin on more advanced behaviours.