Sometimes zombies are flammable, while other times they are not at all (since they are a big squishy wet blob). Have the zombies in The Walking Dead been shown to be more or less flammable as a living human?
It would appear they burn just as well as normal folk. I put the reasonsing for this in the following spoiler box.
When the camp is attacked in the first season (I believe somewhere around episode 3-5), they later burn the "dead". Glenn makes a point that their vicitms (i.e. the dead humans) will not be burnt but instead buried. This indicates that the now-dead undead (to avoid use of the word "zombie") burn equally well as a living human being.
Re: TV series - In the final episode of Season 2 ("Beside the Dying Fire"), the following incident occurs:
Rick and Carl lock themselves in the barn to create temporary safety from the walkers pursuing them, and then decide to douse the floor with petrol, get themselves to high ground, wait for the walkers to get into the barn, and then light the petrol, thereby setting the walkers on fire.
This demonstrates that "live" walkers (as opposed to "dead" ones that have been shot/stabbed in the head, etc. as discussed above by bitmask and Andres F.) appear to be just as flammable as ordinary live humans.
In the TV series, in the season 2 episode titled "Nebraska":
Most of the walkers from the barn, except for Sophia and Hershel's relatives, get burned instead of buried, because they were too many to bury. One of the characters explicitly says "We bury the ones we love, and burn the rest".
Therefore the walkers are flammable, just like human corpses.
and further, in the season 3 episode Prey
A number of trapped walkers are doused with gasoline and set alight. Someone returning to the scene the following day finds several of them still groaning and moving, but burned to a crisp and unable to stand upright (presumably their leg muscles have been burned away)
I think that we can safely say that zombies on The Walking Dead TV show are in fact flammable. They do not however appear to be affected by the flames in the same way that the living are. They appear to not feel pain in the same way which is consistent with all other injuries. The Season 5 premier airing October 12, 2014 demonstrates this clearly.