The link below is to a Dr. Bicuspid article (registration required) that speculates on using a digitally enhanced panoramic radiograph in place of bitewings.
This is becoming a common question; Can a dentist use a digital pan to diagnose interproximal decay in place of bite wings? Or to put it another way is it possible to take extraoral bite wings? The answer, at this time, is NO.
There are a number of vendors working on various systems to make this possible and the article linked below outlines some new research at the University of Texas Health Science Center at San Antonio. However at this time the systems available are inconsistent, usualy require multiple exposures and do not have a high enough resolution to diagnose interproximal decay. So for now ou are stuck with an intraoral sensor.
via DrBicuspid Imaging.