Here's the Mini-Skinny on Carbon Monoxide Detectors:
Why it's dangerous:
CO inhibits your body's ability to absorb oxygen. It's odorless and colorless, and can lead to death in enough cases to make it a s. It is caused by incomplete combustion from an open flame, negative pressure on a home sucking back into the home combustion vapors, or possibly from a blocked chimney.
Who should install them:
To do it yourself you can pick up a battery powered unit for $20-60. An electrician can install an integrated carbon monoxide detector for $150-250. (This is what I have in my house for the reasons below.)
Where to locate them:
At minimum you would like to put one near the bedroom wing of a house. Ideal height is 5' off the ground but for a host of reasons (durability & aesthetics) I say the ceiling is a fine location.
False sense of security:
"Lifetime battery" units last about 6 yrs. More troubling to me is that the sensors on even the best units last maybe 10 years so these are items that require infrequent, but some maintenance.
For a more exhaustive writing beyond my opinions and factoids on the issue of carbon monoxide visit this government site: http://www.cpsc.gov/cpscpub/pubs/466.html