Ref recent Boeing Service Letter (15th april 2008) recommending not to use Dry Chemical Extinguishers (Not only in-flight but also in aircraft maintenance hangars and at airports).
Apart from poorer fire fighting capability, there seem to be signifcant risk of corrossion and damage to airplanes/equipment quote 'All of these dry chemical agents are corrosive and abrasive, and may cause moderate to severe damage to aluminium alloys abd electrical/electronical equipment'
It then goes on to read quote 'The use of dry chemical extinguishers can potentially cause more damage to airplanes and airplane electrical/electronic systems than an actual fire'