Eric,
A few MBA users have been experiencing similar issues, not necessarily while playing World of Warcraft, but using other CPU and I/O intensive applications, check out this question:
Kernel_task on a new MacBook Air
Do you mind trying a few things? I think temperature is the answer. Can you measure the temperature? Also, what other processes do you notice taking up CPU time? Is kernel_tasks one of them?
I think the problem is a combination of the temperature, perhaps a "busted" SMC and poor CPU management by the kernel. My friend has an MBA also and we have both experienced this issue in a variety of situations. I am almost 100% certain that the issue is temperature related, but is also caused by a buggy SMC. I think the SMC is having trouble with the temperature threshold and
prematurely shuts off a core. I stress prematurely because I know CPUs should be able to handle temperatures in excess of 200 degrees Fahrenheit, approximately 100 degrees celsius. The application Temperature Monitor, tells me that the upper limit for each core is exactly 100 degrees celsius. In all my tests I have never passed 186 degrees fahrenheit, which is roughly 85 degrees celsius.
Any additional readings, tests and experiments would be greatly appreciated.