Saw dust is very fine & there is a strong chance of it causing eye or nose irritations. Also, you have no idea what wood the saw dust came from. It could very well be pine, cedar or some other softwood which we are trying to avoid! Definitely unsuitable due to the possibility of eye irritation & respitory problems.
I know a particular brand sells "white wood saw dust" & I've heard some people saying that white wood means its not soft wood. Not true! White wood simply means it comes from a lighter coloured wood like pine, spruce, jelutong etc. Red wood refers to darker coloured woods like cedar, meranti, teak, etc. I wouldn't recommend saw dust even if you intend to cover it with lots of hay or straw 'cos cavies love to dig & they will very likely reach the saw dust.
Poor moisture absorbancy.
Poor odour control.
Very likely to cause respitory problems or eye irritation.