PC-12 on short final executed a go-around when the tower clears traffic onto the runway.

Narrative

Right after conducting my ILS approach; I broke out the clouds at approximately 500 FT AGL; I had the runway insight and verifying the final check items to complete my landing. I heard a Cessna 210 calling the tower ready for take-off; to my surprise; I heard the tower giving the clearance for take-off to the Cessna; so I quickly jumped on the radio; saying: 'Pilatus is in short final'. Then the Controller realized what just happened and called the Cessna to hold short of the runway; but at that time it was too late and the Centurion was getting lined up for take-off. I went around; the controller called me to instruct me of the 'go around' and to join the downwind. Another problem was that in order for me to complete the traffic pattern for landing; I had to remain clear of clouds; which were already low (bases at 500 to 600 FT). So I ended up doing the traffic pattern in the Pilatus at helicopter altitude and capturing the attention of my passengers. Landed without further incident. This teaches us a lesson on being alert for the unexpected at the last minute and verify instructions received by the control tower or our fellow crewmembers who share the cockpit with us.