I did a little google and no they do not have CO2 in them. Air dusters, or at least the ones i looked at, contain a chemical called Difluoroethane, and yes it is compressed into a liquid. Here is a little info on the chem


General
Synonyms: difluoroethane, Freon 152, Freon 152A, halocarbon 152A
Use: aerosol propellant, refrigerant (use now largely discontinued)
Molecular formula: C2H4F2
CAS No: 75-37-6
EC No: 200-866-1

Physical data
Appearance: liquefied colourless gas under pressure
Melting point: -117 C
Boiling point: -25 C
Vapour density:
Vapour pressure:
Specific gravity: 1.012 g/cm3 at 26 C
Flash point:
Explosion limits:
Autoignition temperature:
Critical Temperature: 113.7 C
Critical Pressure 44.9 atm.