While Russia can't get away with using their military to strike Estonia--a NATO country--I'm not sure that Russia made a wise choice here. It seems to me that ultimately, the Russians are playing to their foe's strength:
Estonia—"e-Stonia" to its fans—practices forms of e-government advanced even by Western European standards. Estonians pay taxes online, vote online, bank online. their national ID cards contain electronic chips. When the country's Cabinet meets, everyone brings their laptop. When denial-of-service attacks start taking down Estonian Web sites, it matters.
Even if NATO offers only defensive help to resist these Russian cyber-attacks, if the Estonians are as wired as this article says, the Estonians will have the skills to strike back at Russia on an even playing field.
Remember, in cyberspace, nobody knows you're a dog--or a little country like Estonia. Putin may have picked on the wrong country.