Igniting entire forests? although resisting my urge to point out a flaw in the reasoning I must anyway...
As we should know, most forests are not coated in oil, and such should not spread excessively quickly nor too far. Most forest fires occur during dry seasons because the wood and foliage is dry and easy to ignite, causing a chain effect.
And, taking into consideration that most of Skyrim that we have seen so far is foggy, wet, and covered in snow, the fires should not spread far anyway. The fact that Skyrim is mountainous would also mean that it is high in altitude, further meaning that there would not be too much oxygen in the air. Since fire needs oxygen to burn, it would be further constricted in an environment that has little of it.
Although burning buildings down sounds like fun, it would be impossible if the game uses interior cells (which it will). Since the inside of the building is not actually made into the building on the outside world, we would not be able to see into the building of it caught on fire.
Therefore, we are more likely to see buildings get scorched and charred on the outside as an additional texture, rather than burning down completely.
And one more thing. The development of independent fire, or fire that behaves independently and is not controlled by a loop animation, is a fairly new physic in games. suffice to say, even a small fire that moves independently would be a hit to performance. My computer would surely be overwhelmed even with the thought of trying to process an independent fire that is the size of a forest or region, let alone just a house.
In short, it is unlikely to see fire being used on such a large scale, perhaps in TES:VI or VII. It is more likely to see small fires on the ground or on surfaces, and then dissipate quickly, leaving a scorched ash pile in it's place.