See attached screenshot. A flat surface of wood never catches fire from lava placed on top of it. I've left a world open like this for over an hour with nothing catching on fire.
I've confirmed that doFireTick is set to true. If I create a one block hole and put lava in the hole, the surrounding wood will quickly catch fire. If I put a block of wood at the same Y level as the lava, it will eventually catch fire, and then set the rest of the wood on fire.
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This is emergent behaviour due to the way that lava sets flammable blocks on fire.
There is a specific area of blocks above a lava source, namely the 3x3 directly above the lava block, and 5x5 on the next layer up: http://i.stack.imgur.com/1vNGB.png (Fire blocks have a similar area, but it is a different shape)
In technical terms, any air block in this area, which has a flammable block beside it, will be converted to a fire block when it receives a random tick (note that the flammable block does not have to be within this area).
Now, looking at your screenshot:
[media]Comparing this to the valid set of flammable blocks, you will note that none of the wood blocks in your screenshot are next to a block that is valid to become fire. Also note that the top surface of the logs are covered by flowing lava, so they are not valid to become fire blocks.
I am not certain on the rules here, but I believe flowing lava will not set flammable blocks on fire, except for falling lava (created when flowing lava drops down one block - this is why breaking a wood block and letting the lava flow down will set the wood on fire).
In conclusion, I'm going to say that this behaviour, while counter-intuitive, is by design.
Lava is being more likely to set wood above it on fire even when they are separated.
A wooden walkway in a cave was set on fire by a pool of lava below it, but separated by 8 blocks of air. A mansion was set on fire, apparently by a pool of lava below it and separated by at least 5 blocks of stone,
Was the behavior of lava supposed to have been changed in the update?
Confirmed for 14w18b. You need to break some blocks.