Edit: As some of you have stated, donating corrected voicemails on a per-voicemail basis was previously possible, and those messages would then be analyzed by a person. This new feature is a toggle that shares all of your voicemails with Google, which are then analyzed by a machine, not people. So, this will presumably result in much faster improvements, and hopefully much better accuracy, by using a much wider data set.

Has Google Voice ever screwed up a voicemail transcription for you? Of course it has, because if you've used Google Voice and received a voicemail, you are guaranteed to have received an admittedly hilariously error-riddled paragraph (such as these shining gems of accuracy) accompanying said voicemail.