Once in a while if I make several consequent edits to a post on SE sites I encounter captcha. This never happens to me on Stack Overflow, although I use the same Open ID on each SE site and behave mostly the same way except that most of my activity is concentrated on SO.
To me it looks like captcha tracks my activity on SO and thinks that "it's allright, that user does such thing once in a while, no need to confirm he's not a robot", but captcha on other sites doesn't know about my usage patterns.
Guess what? All SE universe sites are equal to me - I will answer a question on SO and then go ask a question on DIY SE. To me separate sites are like separate directories on the same hard drive each storing certain kind of content.
It would be very good to have captcha that would identify valid user patterns universe-wise, not per each site.