Fax spam is already illegal, under a different set of laws than the DNC, the idea of which is that unsoliticted marketing that costs the receiver (in toner, ink, paper, etc.) is illegal. Good luck though, the law is never enforced to my knowledge.
edit: btw, the DNC specifically does not apply to fax lines or ANY number used for business or commercial use. I laugh when I see that people who have a business in their home have signed up their business line for the the DNC (I see this with RE appraisers all the time). Sorry, Charlie...