Per a Shimano rep, their 10-speed STI shifters wind the cable taken up tightly inside and the cables are prone to breaking if not replaced regularly. My 5600 105 right (rear) shifter broke the cable inside the shifter after 3800 miles, then again in another 4400 miles. I now replace it regularly every 3000 miles and have still had a few broken strands at that point on maybe a third of my replacements.

It's not just that one shifter of mine. I'm on my third 105 right shifter, having worn out the pawl/ratchet gear on the other two after about 16,000 miles each. They have all been hard on cables; it wasn't just one aberrant set.