I can see a market for buffering devices that allow power through, and perhaps do power negotiation for you, but that do not allow data traffic. I believe these devices already exist. though I don't know how sophisticated they are.
However, manufacturers are already realising that blindly trusting every USB host is a bad idea. For example, iPhones now ask if you want to trust if you connect to a previously unknown computer via USB. If you chose not to, your iPhone will take power, but won't allow data access. Therefore it's no longer possible to create a device that looks like a charger but that actually downloads all the photos from your phone.
Both FireWire and Thunderbolt offer DMA. There's publicly available tools that allow you to extract passwords, remove lock screens and find keys in the memory of running devices using either type of port.
Turns out OS X 10.7 and later at least disable DMA access while the computer is locked (unless fast user switching is turned on).
[1]...if you plug untrusted devices into your expansion slots or leave your laptop unattended.
But keep in mind that this is something that a host actually has to turn on for every single device on the bus[1], but probably most drivers are lazy and just unconditionally enable this globally.
[1] See PhysicalRequestFilter in the OHCI specs http://download.microsoft.com/download/1/6/1/161ba512-40e2-4...
For hostile / high security environments, I'd rather have it in a special charging-only hub (good luck pwning such a device) that physically lacks the data wires on the output socket.