The good news is that the problem appears to be intermittent, at least judging by replies on the Google forum. The bad news is that a Google support representative said they were investigating the problem hours ago and there hasn't been any update. Just know that if you're using Google Voice that you could run into some issues getting message replies for a bit -- perhaps reach out to your contacts on one of the many better-supported messaging services out there to make sure you're not missing anything.

Update, 2:30PM ET: Google confirmed the issue and says it has since been resolved, though it is continuing to monitor on-going reports from users.