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) .
Newer smoke detectors, as well as carbon monoxide detectors, are photoelectric, and have no radioactive components.
If they contain a non-alkaline battery, the battery must be removed and recycled properly.
They 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 recycle smoke detectors for $8 each, if you mail them in.
You may also send ionization smoke detectors back to the manufacturer (addresses here, scroll down).