If you want to solve the GPL compatibility problem cleanly, why not dual-license? Then, you can use Larry's form of termination. If anyone litigates against free software, the Apache 2.0 license terminates and they are left with the GPL as their only valid license. Then, if it happens that they had previously mixed proprietary software with Apache, or they depend on any such software, that gets poisoned.
Because with dual-licensed software it is impossible to know what terms are being followed by any given contributor, short of an explicit declaration on every contribution.
....Roy
