David Richardson has created a multiple-part tutorial for modeling a mech in ZBrush.
Rafael Grassetti - Design and Anatomy Package
Check out Rafael Grassetti's Design and Anatomy package, with over 5 hours of ZBrush character anatomy and design tutorials.
ZScripting with ZBrush - Eat 3D Tutorial
Eat 3D have released a video tutorial on ZScripting with ZBrush.