A great alternative to thinking about things in terms of CAP that Justin brings up is harvest-yield, where yield is the probability of completing a request and harvest is the fraction of your data that the response actually represents. Here's the paper:
http://lab.mscs.mu.edu/Dist2012/lectures/HarvestYield.pdf
hmm, i'm not sure how it could be better worded, but since antirez already links to this, i had thought you were posting a response to antirez's comments
http://en.wikipedia.org/wiki/CAP_theorem