When we moved into our new home in '07, it came with three CO2 detectors (1) on every level of the house they are all hardwired w/battery back up and they are also smoke detector combos, we have smoke detectors in each of the three bedrooms.

Carbon Monoxide (CO) has approximately the same vapor density as room air, meaning that it will hang around at the vertical mid point of a given space. The CO detector should ideally be installed at the four foot level, assuming the ceiling height is eight feet. If you only have one detector, I would put it in close proximity to the bedrooms. If your residence is multilevel, I'd put one on each level. Additionally, keep in mind that the CO sensors are usually good for only about 18 months. If the detector is located in an area where it would receive above average amounts of CO, such as garages, fire place or wood burning stove areas, etc. the sensor lifespan could be shorter. Check the manufacturers information for additional details and specific mounting requirements.

CO tends to disperse quite evenly through a building so you really needn't have more than one per floor and/or group of bedrooms. The biggest concern is to avoid dead zones where air stagnates. Keep CO and, smoke detectors for that matter, at least 12" away from inside corners, doors and away from areas likely to be covered by drapes, furniture and from behind doors that might block air flow.

Around here CO detectors get mounted about knee height and typically in the hall and outside bedrooms as this keeps them out of the way and out of sight lines. It also limits the tendency of people to put furniture in front of them. But as long as you keep a decent distance away from dead zones height is not critical.

CO poisoning is usually a slower process compared to fire and, because of its tendency to mix and spread, usually large areas of the house are effected. Which means that unless there is a potential source of CO within a bedroom, perhaps a small gas heater, then it really doesn't make sense to have one in each bedroom.

As long there is good coverage more CO detectors are not necessarily better. Not if buying more means you buy cheaper ones. And not if after ten years or so, when the CO and smoke detectors need replacement, you put off replacing them.

CO.
Not C02

CO = Carbon Monoxide, a deadly byproduct of combustion.
C02 = Carbon Dioxide, the stuff in soda that makes you burp.

Drives me batty when those are mixed up.

I have 3x CO detectors. We also have hard-wired smoke detectors.

1 in the master bedroom hallway (all bedrooms connect to this hallway)
1 in the basement (we have a wood stove, need to know ASAP if there's a problem there)
1 in the kitchen/dining area (we have a fireplace, that area needs coverage).

A few years ago we had ALL THREE of them go off at once when we got home on a cold night after a few days away, and lit fires in the wood stove & fireplace at the same time. The oil burner kicked on just as the fires were being lit. The slight negative pressure in the house caused by the oil-burner coupled with very cold-pipe chimney caused a momentary reversal of the draft and pulled CO into the house. BEEP BEEP BEEP BEEP - scared the crap out of me, as at the time I had no idea what the hell was going on, there was no odor at all.

CO = binds in the same place on hemoglobin as 02 (oxygen ). This is bad.
CO2 = binds in a different place on hemoglobin than 02 (oxygen). Not bad.

Hmm... maybe jogging alongside the road isn't such a great idea.

http://en.wikipedia.org/wiki/Hemoglobin

I'm a Code Enforcement/Housing Inspector for the largest city in VT. We deal with this all the time. Many of the new building/electrical codes require the installation of hardwired smoke detectors in new construction or at the time of sale. Depends on the area you are located in and whether or not your local government has adopted the newest building or electrical code.
Sounds like your detectors are interconnected. That's a good thing. You want the detectors in your bedroom to go off at the same time as the one in the same location as the fire. Conversely if the CO detector in the basement goes off it will activate the other CO detectors in the house. Giving you time to get you and your family out. You can purchase combonation smoke and CO detectors. They cost about 75 bucks. Put one in the hallway outside your bedrooms, and one on each level of your home. The one outside your bedrooms counts as that level. Make sure that the electrician you hire to do this follows the instructions. There may be a minimum distance that the detectors need to be located from fuel burning appliances. Ceiling location is usually fine as the gas is very similar to air and warm air rises. If you are still concerened then by all means use a plug in type of co detector. Check to see if the detectors are UL 2034 listed and approved. Good luck, seems your are certainly on the right track
Matt