Interesting. Are there many soviet Russians still in Estonia who can differentiate /s/ and /z/?
There are also people, who paid attention in the first grade when alphabet was taught.
The differences are also taught in Russian classes (5th grade).
Estonians usually do not pronounce z, š and ž correctly because they think they would sound too fancy.
In other finno-ugric languages (especially Karelian) they use these kinds of sounds. (http://en.wikipedia.org/wiki/Karelian_language#History)
For example mees is mieš in Karelian.
In Finland I saw a newly published book called "Livvikarjalan kielioppi". It was quite cool.
I can give some examples of Karelian languages in my later posts.