Very misleading article title... It doesn't really "break into" the computer. Basically, all it can do is grab your cookies. Big deal.
Also, there's an easy fix (in OS X at least): Make sure your main network device (Ethernet or Wireless) is dragged to the top of the priority list (this is the default) in your network settings. That way, when this "PosionTap" is plugged in, the system won't try to route internet traffic across it.
The hack is also dependent on you having a website open that's making Ajax requests or refreshing iFrames. On OS X, by default when your system is locked, Safari suspends all web processes, so I'm not even sure this would work... (Now if they attached it and somehow hid the device, it would work as soon as you unlocked the machine.)
This "hack" is dependent on *a lot* of very specific circumstances.