Smoke and carbon monoxide detectors may be disposed in the trash.
Older ionization smoke detectors containing the the very weak radioactive isotope Americium-241 are not considered hazardous (click on "smoke detectors" at MassDEP site) .
Many brand owners will accept them back if you mail them. Click here for company names and addresses.
Newer smoke detectors, as well as carbon monoxide detectors, are photoelectric, and have no radioactive components. They may be recycled with electronics.
If a detector contain a non-alkaline battery, the battery must be removed and recycled properly.
Detectors are not made in a way that can be easily deconstructed to their components, which is necessary for recycling any product. Curie Pack's website says they will deconstruct and recycle smoke detectors for $8 each, if you mail them in.