It would seem that there is in fact a way to force an evolution of your Eevee. The Eevee Brothers from the original anime were named Rainer, Pyro, Sparky and they owned a Vaporeon, Flareon and Jolteon respectively. If you nickname your Eevee either Rainer, Pyro or Sparky, your Eevee will evolve into that desired Eeveelution!
Sparky = Jolteon
Rainer = Vaporeon
Pyro = Flareon
Whilst there has been a number of confirmations of this working we need your help to confirm this! So get evolving people.