As for the science inside the breathalyzer, it works like this:
Cr2O72- + 8H+ + 3C2H5OH --AgNO3--> 2Cr3+ + 3CH3COOH + 7H2O
Before the test is used, the acidic dichromate solution is yellow/orange, due to the dichromate ions. However, after ethanol is added to the mix, it reacts to produce green/blue chromium(III) ions. A spectrophotometer can be used to compare the reacted sample with an unreacted solution to determine how much alcohol is in the person's blood. The amount of alcohol in 2100 mL of breath is equal to the amount in 1 mL of blood.
Info from a Chemistry 30 class and doublechecked at http://classes.colgate.edu/rbowman/core145/Crime_and_Chemistry_Laboratory_F00.pdf