I've looked everywhere for the answer to this. And it is honestly a important question. Is the iPad Smart Cover flammable?
No. In your case of the leather cover - I can't imagine any circumstance where someone would call leather flammable. (or inflammable or combustible or any other word which means "easily set on fire")
Leather is what blacksmiths, welders and others working in hot or hazardous environments use for flexible protection when asbestos or other fireproof materials are overkill.
I haven't seen a good chemical analysis of the makeup of the polyurethane cover, but in general urethanes begin to break down chemically at the same temperature that paper burns, so even if it doesn't ignite, you might not want to expose your smart cover to burning temperatures.
Neither is going to ignite easily or need handling typical of a "flammable" substance.
Pretty much everything burns given the right circumstances, but definitions of flammability rest on a lot of assumptions about specific environmental conditions. If your question boils down to, "Under what circumstances can you expect to fear combustion on the assumption that the cover is attached to an iPad?", then we can proceed with some well-reasoned speculation.
I would say that if you are keeping in line with the operating requirements of the iPad itself, then without exposure to naked flame (which would likely fall outside said requirements) I can't see a) how it could catch fire, and b) why you would be worried about it rather than the iPad...
So, assuming that you haven't managed to get a bulk load of smart covers that you intend to insulate your central heating pipes with, what is your specific concern with regards to flammability?
Also, we need to take into consideration the colour of your smart cover. As you know, darker colours absorb heat, and lighter ones reflect it. A black smart cover being used to protect your iPad from a very naughty child with a magnifying glass and a line of sight to the Great Glowing Orb in the Sky (or GGLOITS as we worship it round here) may be able to acheive combustion more quickly than if you used a white one. Also, firing that bad boy up and playing severals hours of Infinity Blade 2 whilst charging the battery might kickstart the combustion process if you can get if running hot like a Macbook running Flash Adverts.
But then we should not forget that the smart cover is more than just a nice piece of bendy screen protection, but has magnets and metal parts as well. Will these burn, or will they merely melt?
So, yes, getting silly now, but figured someone had to do it.