One thing I'm curious about -- if you want to spread your instances over multiple zones and you are using postgresql, how do writes work? Won't latency to the master be slow if instances in one zone are trying to write to the master located in the other zone?
Latency between different availability zones in the same region is generally pretty good. "Over the last month, the median is 2.09 ms, 90th percentile is 20ms, 99.th percentile is 47ms. This is based on over 250,000 pings -- one every 10 seconds over the last 30 days."
from http://www.quora.com/What-are-typical-ping-times-between-dif...
Certainly 10% being 20ms or more is a little troubling, but if this is only for writes (ie reads come from a slave in the same AZ) you are probably ok.
