So I had my UNO R3 plugged in a 12V 5A brick as well as USB connected. I was running a little test circuit set up on a bread board. Whilst reaching over the breadboard I knocked the 5V line loose which promptly planted itself into a GND slot. Lucky me... The Regulator failed closed circuit with a good pop. I can measure a short from Vin to Vout. So I had 12V on my 5V line. Which of course fed straight back to my laptop via USB. Unfortunately the laptop became an instant paperweight. Not a flicker of life. Even the charging light no longer works. I assumed there was a protection diode to prevent that. Expensive lesson learnt.
I hooked the UNO onto a USB charger, surprisingly things there seem to have fared somewhat better. At least the 328 seems to have survived. But I noticed directly after the incident the little atmega16U was pretty hot. from memory I believe that one is involved with USB serial conversion or something along those lines. Now I'm worried about plugging it into my desktop in case that chip is borked too. I can't afford to fry another computer.
So given that things "appear" to still work, how real is the risk that potential damage to the UNO could cause damage to a computer now? Is there a way to test it?
Thing that pisses me off most is the 12V adaptor didn't even need to be there. It was only connected because I ran the circuit overnight to test stability. I could have disconnected it this morning. 
Cheers.