[shabble]

There certainly are, although they're not typically intended for home usage. But in industrial/commercial setups which use CO2 (beer/drink-cellars, dry-ice handling, pressure pipe-flushing, etc) they're relatively common. Obviously they're always detecting some level, but they're set to trip or alarm when concentration hits some threshold over ambient. http://duomo.co.uk/CO2_Alarms.aspx is just one example.

Low-O2 detectors are also common, and approach the problem from the other direction. They're more useful for N2 or other inert gases, since you're less concerned with toxicity and more with plain asphyxiation. IIRC they're used in some places to enforce "No riding in the same lift as the liquid-N2 dewar" safety rules.