I had no trouble at all implementing an interface and several plugins, following Trolltech's examples. They work perfectly. The problem here is trying to implement something akin to Java-style "adapter classes," where an intermediate class with implementation of the pure virtual functions declared in the interface is used to ease creation of inherited classes.
I suspect that the Qt framework only allows inheritance directly from the abstract base class. At least, that seems to be the case in practice, as noted.
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