There have read some speculation about whether this is possible or not. The hard part (as I understand it) is that you need a real time driver for whatever you want to control in a real time fashion. USB was never designed to be a real time interface. Mesa has a card that works over ethernet, which may solve a lot of the issues that typically drive people to USB.
I hear a lot of people on either side of the Mach/LinuxCNC debate (might I call it a "holy war") say Mach is superior because it can use USB, or Linux is superior because it uses a hard real time kernel, and USB is crap. Ultimately they are two very different pieces of software designed in totally different ways to do approximately the same thing. Keep that in mind. For example, SmoothStepper doesn't work with LinuxCNC, which is a gripe, but if you have a smoothstepper, use Mach; it's what the SmoothStepper was designed around. If you want to use LinuxCNC, then don't buy a smoothstepper, for hardware acceleration on LinuxCNC, Mesa cards are a better choice (I recently upgraded, and couldn't be happier).
As far as getting USB support in LinuxCNC, maybe it's possible. Would performance be good enough to make it worthwhile? I don't know. I have a feeling somebody has done it, even if it is not in the official version.
I still think these little cards are too underpowered for a mill or lathe, though they might be interesting for a 3d printer or hot wire cutter.