Is sawdust really that bad for my vacuum cleaner?
I can't really see why saw dust is any different than the dust you find around your house (mostly made up of dead skin cells).  The only real difference is that you tend to clean-up a lot of saw dust at any one time, whereas it takes a long time to fill a vacuum bag with typical around-the-house cleaning.

The dust should never come in contact with the motor.  I think you're more likely to damage the motor by running it with a clogged filter (most vacuums since people tend to never clean them) then you are cleaning up saw dust with a clean filter.