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. This week, that's finally changing, according to Googler Alex Wiesen.
Starting this week, just go into your Google Voice settings on the web, and you'll be asked if you want to anonymously share your voicemail messages to make transcription better. As more people turn the feature on, the better transcription will get. And, sadly, probably the less hilarious it will get. Oh well, it was fun while it lasted, right?
As an aside, this would appear to be different from the existing "donate your voicemails" system, which presumably only analyzed voicemails as part of Google Voice's transcription engine. It also only worked on a one-by-one basis - you couldn't just have a checkbox permanently ticked. If I had to guess, this new feature is probably better integrated with the full Google speech recognition engine power products like Now and the Google keyboard.