bibtex @string feature

Sorry, this feature is not perfect yet. You need to type string as all upper case ("STRING"), and the key of the string will be made lower case. Sorry, we know this isn't documented anywhere yet. The good news is that with those changes your example seems to work:

%---------------------------------------------------------------
% strings de nomes usados em bancas
%---------------------------------------------------------------


@STRING{Andre = {{Andr\'{e}} L. P. Guedes}}
@STRING{Murilo = {Murilo Vicente Gon\c{c}alves da Silva}}
@STRING{Vignatti = {{Andr\'{e}} Luis Vignatti}}

@MastersThesis{Martins2012,
  author =       {Geoffrey Alberto Vitorio Martins},
  title =        {Manutenção de Caminhos Mínimos em Grafos Dinâmicos},
  school =       {PPGInf/UFPR},
  year =         {2012},
  month =        {agosto},
  day =          {30},
  type =         {Dissertação de mestrado},
  note =         "Orientador: " # andre # ". " #
                 "Banca: "#
                   murilo # " e " #
                   vignatti,
  keywords =     {OrMScC},
  url =          {http://hdl.handle.net/1884/35025},
  pdf =          {http://dspace.c3sl.ufpr.br/dspace/bitstream/handle/1884/35025/R%20-%20D%20-%20GEOFFREY%20ALBERTO%20VITORIO%20MARTINS.pdf?sequence=1}
}

gives:

Manutenção de Caminhos Mínimos em Grafos Dinâmicos. Martins, G. A. V. Master's Thesis, PPGInf/UFPR, agosto 2012. Orientador: André L. P. Guedes. Banca: Murilo Vicente Gonçalves da Silvae André Luis Vignatti