You also may want to check the reports that having ionization detectors is not good enough.
http://video.today.msnbc.msn.com/today/49269880/#49269880
Not always the case but typically, the 120V linked smoke detectors for a home are the ionization type. Quick to react to nuisance alarms like over cooking your bacon, but they may be slower to react to smoldering fires. It isn't a coincidence that ionization alarms like this aren't typically monitored; they cause many false alarms.
On the other hand, typically, monitored alarms are the photo-electric variety. Less false alarms but usually genuine alarms and more sensitive to smoldering alarms.
Like others have said, there is lots of confusion out there, and unfortunately cost and/or aesthetics sometimes seems to be winning over safety. If you fall into the camp that thinks just adding a relay to your required building code smokes, and connecting them to your monitored panel is enough, I would think again. You might want to do some searching but it won't take you long to come across the stories of how people have died just relying upon ionization smoke detectors.
While there is certainly much debate about this, the widely accepted method is to NOT touch those interconnected smokes required by building codes. Instead add a second layer of photo electric smokes designed to be connected to a panel, and connect them to your panel. While your at it, add some heat-rise detectors in your attic and kitchen and furnace room, and maybe a Carbon Monoxide detector or two as well. And remember Carbon Monoxide detectors only last 5 - 7 years, and ionization detectors should be replaced every 10 years.
The debate will continue, and its your house so you can do what you want, but just fully understand the actions you take.
