I found this on another VW group forum:
"I now have some more information about fast idling with the TSi. Dealer "A" told me the fast idle probably happened because it sensed the battery voltage was low. Considering the car is only a year old and its the middle of summer this seemed entirely unlikely. Dealer "B" told me it was to get the heater working quickly. Or maybe the air conditioning. A detailed search on the internet provided a third and most likely explanation which I have pieced together below, this applies to the 1.4 TSi CAXA engine but may also apply to the 1.2 TSi.
After starting, the Engine Management Unit looks at various temperature and other sensors and decides if it would be safe/worthwhile to pre-heat the exhaust catalyst to reduce emissions. Rather like Goldilocks and the porridge, if things are too cold or too hot the answer is no. If the answer is yes, the idling revs are increased to around 1300 and the ignition timing is considerably retarded. The late ignition means that the gases coming past the exhaust valves are still burning hotly, which raises the temperature of the catalyst more quickly than would happen otherwise. It also makes the exhaust sound wierdly burbly like a diesel. After about a minute, the revs are gradually dropped to normal and the odd noise stops. Unlike older VAG engines, there doesn't appear to be a separate whining air pump for the warm-up cycle, possibly (my theory) because direct injection can be leaned off immediately after starting and there is therefore less carbon monoxide needing extra oxygen to burn.
In principle the car is still drivable while this is going on, but in practice it would seem that the engine management has some difficulty coping with all possible drving conditions and jerky running and spitting are not unknown for the first few hundred yards, especially if you go above 2000 rpm. I imagine this would be "interesting" if you have DSG. From now on I am going to confine myself to shutting the garage doors, putting on the seatbelt, twiddling the radio and slow speed manoeuvres until it has "done its stuff".