Are mining pools evenly distributed across the planet?
Evenly distributed, no.  

There are pools on nearly every continent.

But a pool only has the hashing power of the individual miners participating.  Miners can (and do) jump from one pool to another when connectivity issues occur, or when workload isn't received, or when payouts stop.

Individual miners are much more spread out. Areas with cheaper electricity got higher participation than areas with lower electricity, but now with ASICs that will probably not be as much of a factor for a couple years again.