From the Guardian:
While Russia and Estonia are embroiled in their worst dispute since the collapse of the Soviet Union, a row that erupted at the end of last month over the Estonians’ removal of the Bronze Soldier Soviet war memorial in central Tallinn, the country has been subjected to a barrage of cyber warfare, disabling the websites of government ministries, political parties, newspapers, banks, and companies.[...]
While planning to raise the issue with the Russian authorities, EU and Nato officials have been careful not to accuse the Russians directly.
If it were established that Russia is behind the attacks, it would be the first known case of one state targeting another by cyber-warfare.
Considering that both Estonia and Russian are infamous for the number of nefarious online souls (crackers, spam/warez-net operators, etc.), and considering that I have the general impression of some of those folks being rather…enthusiastic about their pet causes, I would actually bet against these attacks being undertaken by the Russian government.
There are enough quasi-independent actors in that part of the world to provide a simpler (and perhaps more difficult to resolve) explanation.