Does anyone use non-rewritable media for electronic voting? If not, some of the same issues are just as applicable to other systems as to Diebold's. To be sure, the succeptibility to outsider cheating may not be as bad on other systems as on Diebold's, but any system with rewritable code will be subject to undetectable insider cheating.
By contrast, putting code and votes on non-rewritable media would mean the only way to cheat would be physical substitution of the media in question. Use of well-designed serialized holographic seals could make such substitution sufficiently difficult as to no longer be the easiest method of fraud.
Non-rewritable would be safer, but not absolutely necessary. You could still transmit a virus with it, and the best safety it gives is the inability to change votes, but in this case the votes put on the media are already bad if the machine is infected.
And we could achieve the same thing with flash cards if the machine would just cryptographically sign the vote file.