Peter over at Applied Inspirations has a very detailed article on replacing the Hexbug’s primitive brain with a PIC microcontroller. The article is very detailed, complete with schematics, code, and a link to purchase the printed circuit board ($12.50 shipped).
I’ve disassembled a lot of bugs, and even tried a dual motor hack which almost worked. This should inspire me to take another try at it and see if I can work out the friction issues. The Inch worm should be fun to hack since it already has two motors and an IR remote.