Recently, this issue has been discussed again. I can only say that in my games I've never stolen a single tech in spite of having "advanced" espionage level with some or all the ai opponents.
I've seen people report that they have stolen technologies from ai's, so it should be possible, shouldn't it?
Reasons for me not stealing any technologies could be:
1. I return espionage spending to 0BC after I hit advanced level. May be you have to continue to spend money to steal any techs.
2. In recent games I played with tech trade off, so may be you cannot even steal technologies apart from invading enemy planets. But I dind't steal any technologies with tech trade on before...
3. a) People reporting technology stealing may have gotten the technology due to invasion of enemy planets - not "real" stealing due to espionage. In effect this would mean, there is nothing such as genuine technology stealing. -->

This is supported by the fact (a fact in my opinion, that is) that the "techs stolen" overview at the end of the game really shows techs stolen due to planetary invasion. Normally, the Drengin and Yor are good in this part of the overview, because they invade many ai planets.
I cannot decide on this issue, I can only report that I never genuinely stole a tech despite hitting advanced espionage status sometimes very early on in the game. What are your opinions/findings?