I have a fair bit of maintaining and operating experience now with both certified and uncertified Rotax 912 motors in the UK.
When the engines are cold and the air humid, the certified engines (with exhaust muffler fed hot air to the single air box) can be seen to pick up a bit of ice. I have never seen this in the air - only during maintenance and pre take off ground runs.
On our RV-12 we fitted an excellent (non-certified) carb heat system:
Skydrive Rotax 912 Carb Heat
The great thing with this set up is its simplicity. Coolant is fed to jackets mounted on the outside of the carb outlets. Carbs are kept warm permanently when the engine is running but there is no loss of power as the air remains cold.
This is a very popular system in the UK where we do get the occasional damp/humid carb ice inducing weather....