I'm starting to see USB ports in regular wall outlets. They look like they are significantly more expensive than a wall-wart but much more tamper-proof.
There's two downsides:
- The USB ports are more easily damaged than regular wall outlets if someone trips over the cord or bashes the plug with the back of a chair.
- People will unplug your devices to charge their phones.
Without having the whole thing enclosed inside the wall, it's difficult to keep it safe. A future electrician may accidentally put mains power onto your wires and then you have unregulated AC on wires with only 5V worth of insulation. That's why it is rare to see low-voltage wires coming out of a wall socket and when you do see it, it looks like the worst kind of hack job.
I do like Paul's suggestion: use phone jacks. They are built for wall mounting and are easily replaced if they get smashed. Be aware that someone may find a real phone outlet (or Ethernet) to plug your device into, so you've got to expect 48V AC as a likely fault condition that your device must withstand without bursting into flame.