The antistatic amine additive in Ethafoam makes me a little concerned for using it as a long-term storage option. The devil is always in the details, and your link doesn't mention what specific amine is used for antistat, but different amines (ethoxylated, non ethoxylated, aromatic, aliphatic) have different levels of reactivities and different Kb values (i.e., pH). Most amines that would be used for antistat have boiling points in the 180-220C range, meaning that there will be enough vapor pressure to to have a non-zero amine vapor concentration capable of condensing on your mics. Amine-free antistatic bags are marketed for storage of sensitive electronic equipment. Amines will react with some plastics, most notably polycarbonate. It's conceivable to me that there is a slight chance that the amine might be able to get to the adhesion promoter between the gold and the PET membrane and speed up the delamination of the gold from the capsule. Possibly worse, is that condensing amine on the high impedance parts of the mic may cause leakage paths to ground (on both capsules and PC boards). This is actually what the additive is designed to do, for static dissipative purposes. Here's the possible scenario: The amine vaporizes in the mic case, it condenses on the capsule/PCB, and attracts a monolayer of moisture to those surfaces, reducing the impedance of these sections from gigaohms to megaohms. The same caution would apply to storing your mics in antistatic bags with amine based antistats.
I don't want to sound like I'm overselling the dangers of using Ethafoam or antistatic bags. This is just a possible scenario, one that might be way less damaging than using an alternative foam that outgasses and decomposes. My speculation is a nice thought experiment, but a call to Dow chemical might get a definitive answer. If you contact them, make sure they understand about the ultra high impedance circuitry used in microphones, and that 10^6 to 10^10 ohm/sq might be too much of a leakage path for this circuit.
-Chris
