We seem to have the same problem. certificate is from tawte. Older Air Apps are can not be installed any more, as far as i tested.
I never heard about expiring signed AIR apps. Is this a bug or a feature? Who can help?
We use AIR since AIR 3 and have many older Apps out there that are all signed by certs of tawte. Some of them are sold on DVD. So this is a big problem if its true that all theses older apps can't be installed any more.