The A.O.U. voted against the proposal, citing the need for more data. For now, these groups of nuthatches remain subspecies, and they vary in plumage, bill shape, and vocalizations.
Now when I see and hear White-breasted Nuthatches, I pay a little more attention. It's a chance to consider how they might vary, and it's also an opportunity to contribute to what's known about them in different parts of the country.
Here are two views of a male White-breasted Nuthatch in Santa Rosa on 14 March 2015.
Examples of things to look for: the length of the bill, the amount of white in the face, the width of the black crown, and the amount of black in the blue-gray feathers.
And you can listen to this male singing, too. I should have also tried to record the call notes (which vary among the subspecies), but I have a brief recording of his song. [By the way, I haven't heard a discussion about why the call notes would differ more than the song. Nuthatches do tend to call a lot, and they don't sing that often. Could that be part of the reason?]
Pardon the background noise, as this recording was made with my camera. Remember to turn up the volume of your speakers.
If you're interested, you can read more about differences in the subspecies of White-breasted Nuthatches in this article by Steven Mlodinow.