A physical volume control device doesn't need power.
Wi-Fi needs power.
You can't pull the power off an amplifier (and the amp may not even be on) to run a Wi-Fi connection to your home... or perhaps the right device could.
I would say that a power switch location can be replaced with another product, such as Insteon or some other power line product and that product could then be interpreted by a control system into a volume control scenario.
On the other hand, I could see a stand-alone company like Russound or someone else making a 'speaker line' volume control unit which would broadcast something back to a proprietary amplifier or pre-amp system.
You could always get a RF remote like the wall mounted KP-900 from Universal Remote, but it comes back to some of the same old issues...
Of course those questions are:
Why do you need a volume control which is Wi-Fi vs. a RF one, or using another interface such as iPhone or similar?
Why limit yourself with volume only when most places need a lot more control?
How do you magically power this device? Your phone needs a charge every day.
What is it going to talk to via Wi-Fi? It would only work as part of a proprietary system at this point I would think.
I've often considered the KP-900 as a nice in-wall solution to getting source and volume control in some of my rooms if I didn't have a control wire in place already.