After reading Jerry Holways book where he comments on using a bubble level, and reading forums, it appears that using a bubble level is not useful while the rig is in motion, such as going around corners etc.

Has anyone tried using an iPod Touch or iPhone with one of the many DIY level apps such as Tiltmeter Pro? These apps are not affected by the motion of the rig and give a true reading of the horizon level.

I'm sure someone could make a Steadicam version of these apps that has larger text to make them easier to read from the operating position.