Opponents of the individual mandate agree with Judge Hudson that the Commerce Clause does not extend to regulating inactivity:
“But these regulatory powers are triggered by some type of self-initiated action. Neither the Supreme Court nor any federal circuit court of appeals has extended Commerce Clause powers to compel an individual to involuntarily enter the stream of commerce by purchasing a commodity in the private market.” Commonwealth ex rel. Cuccinelli v. Sebelius, — F.Supp.2d —- (2010), at *12.
But I can think of at least 2 examples of Congress requiring all Americans to purchase insurance: Social Security and Medicare Part A. The premiums are collected on wages in the Internal Revenue Code for two mandatory social insurance programs:
Section 3101. Rate of Tax.
(a) Old-Age, Survivors, and Disability Insurance.—In addition to the other taxes, there is hereby imposed on the income of every individual a tax equal to the following percentages of the wages (as defined in section 3121(a)) received by him with respect to employment (as defined in section 3121(b))…
(b) Hospital Insurance.—In addition to the tax imposed by the preceding subsection, there is hereby imposed on the income of every individual a tax equal to the following percentages of the wages (as defined in section 3121(a)) received by him with respect to employment (as defined in section 3121(b)) –
These federal social insurance programs ignore the activity/inactivity distinction – try failing to pay your Social Security or Medicare Part A taxes and see what happens next. You will be compelled to involuntarily enter the stream of commerce and purchase these two insurance products from the federal government.
Orin Kerr raises related questions in the context of child pornography laws.
 The federal government also imposes taxes on employers at 26 USC § 1401 for Social Security and Medicare, and Federal Unemployment Tax under § 3310.