And the DEA page: http://www.deadiversion.usdoj.gov/takeback/
You can search for locations in your area that are set up to receive your unwanted drugs on this page. For me in Omaha, it lists sheriff and police stations and mobile command posts. Solid drugs only are accepted, and meth and marijuana are not part of this program.
If you have unwanted drugs in your house from recent treatment, the EPA recommends that you contact your local waste management service to find out specific disposal instructions for your area: http://www.epa.gov/ppcp/faq.html. The main point - don't flush them!! Here are recommendations from Illinois - you should be able to find your state's recommendations by calling your local public library.
An example of what drugs live on in the water even after wastewater treatment can be found in this 'medicinal salts' product recently highlighted on BoingBoing: http://boingboing.net/2010/09/15/alvisos-medicinal-al.html . (Fact: these are not just drugs that folks flushed, but some drugs live on in our human waste, passing through our bodies without being absorbed or changed, so wastewater will always have some drugs in it.)