It appears that the generally accepted (not always correct though) solution is to let the units burn.
Most wind turbines are installed in relatively isolated areas so the major risks are igniting the surrounding vegitation or nearby turbines. The towers are too tall for fire brigade access so a built in system would be the only real solution.
As stated CO2 would pose significant risks for maintenance personel. I would imagaine that the bottles would have to be installed at the bottom of the tower to allow isolation and replacement as well as on weight grounds.
I don't know how much water would be required for water mist, but again weight would probably be a consideration.
They do appear to burn quite well though!
(just a brief sample from a web search)
Another consideration, as with PV systems, is isolation. If, in the worst case, the tower falls the cables need to be remotely isolated to avoid the other turbines feeding the fault.