There are two major arguments that apply to the "voluntary" nature of the tax system. Number one is the contention that Filing is voluntary, whereas the second one contends that Paying is voluntary, usually these two arguments go hand in hand. We'll first look at the former.
Proponents of the voluntary nature of the filing system point to IRS' own literature which point to the voluntary nature of the system. At some point even the 1040 instruction book made this mention. Additionally, the Supreme Court's opinion in Flora v. United States decided on March 21, 1960 (see page 362 of the decision) which says "Our system of taxation is based upon voluntary assessment and payment, not upon destraint".
Ever since this decision by the Supreme Court those who oppose the tax system have made reference to this statement in order to push forward their contention, however, the voluntary nature of the filing system is to allow the taxpayer to initially determine the amount of tax liability and complete the appropriate form rather then to have the IRS determine their taxes for them. However, US Code Section 6011 clearly states "...When required by regulations prescribed by the Secretary any person made liable for any tax imposed by this title, or with respect to the collection thereof, shall make a return or statement according to the forms and regulations prescribed by the Secretary. Every person required to make a return or statement shall include therein the information required by such forms or regulations."
Failure to comply with the filing requirement can have severe consequences including civil and criminal penalties, including fines and even imprisonment. Should you find yourself under these circumstances it is imperative that you put yourself in the hands of a competent tax consultant or attorney (in the case of criminal charges) in order to resolve the situation as soon as possible.