Gonzales v. Raich, United States Supreme Court, June 6, 2005 [ruling that Congress has the power to criminalize the growth and use of marijuana for personal medical reasons with a doctor's recommendation]. Excerpt from the Opinion by Justice Stevens:

Given the enforcement difficulties that attend distinguishing between marijuana cultivated locally and marijuana grown elsewhere, 21 U. S. C. §801(5), and concerns about diversion into illicit channels, we have no difficulty concluding that Congress had a rational basis for believing that failure to regulate the intrastate manufacture and possession of marijuana would leave a gaping hole in the CSA [Controlled Substances Act].

Read the full text of the opinion here [PDF]. Reported in JURIST's Paper Chase here.