Renault are working on something similar: The Synthetic Jet.
http://pdf.aiaa.org/preview/CDReadyM...V2006_3337.pdf
Green Car Congress: Renault Altica: 44MPG Diesel Concept with Active Airflow Management
http://www.people.vcu.edu/~kmmossi/d...ddox_mossi.pdf
http://depts.washington.edu/cims/pub...actuators1.pdf
It's not strictly compressed air but it is doing the same job. It's also been talked about here without much resolution on the matter of compressor efficiency.
Didn't the P-51 mustang have this technology? It enabled it to go further on a tank of gas. I think there would be a smaller percentage improvement on a car using this but it would be an improvement all the same.
ollie