Biblatex supercite command with use of square brackets (like a reference style in Wikipedia)

Would this work for you?

\documentclass{article}

\usepackage{biblatex}

\DeclareCiteCommand{\supercite}[\mkbibsuperscript]
  {\iffieldundef{prenote}
     {}
     {\BibliographyWarning{Ignoring prenote argument}}%
   \iffieldundef{postnote}
     {}
     {\BibliographyWarning{Ignoring postnote argument}}}
  {\usebibmacro{citeindex}%
   \bibopenbracket\usebibmacro{cite}\bibclosebracket}
  {\supercitedelim}
  {}

\usepackage{filecontents}

\begin{filecontents}{test.bib}
@book{A01,
  author = {Author, A.},
  year = {2001},
  title = {Alpha},
}
@book{B02,
  author = {Buthor, B.},
  year = {2002},
  title = {Bravo},
}
@article{C03,
  author = {Cuthor, C.},
  year = {2003},
  title = {Charlie},
}
\end{filecontents}

\addbibresource{test.bib}

\let\cite=\supercite

\begin{document}

We are citing \supercite{A01,C03} and \supercite{B02} and \cite{C03}

\printbibliography

\end{document}

You can comment out \let\cite=\supercite if you don't want \cite to behave like \supercite (but I thought you might, seeing as it is shorter to write).

For information, what was needed is added \bibopenbracket and \bibclosebracket around the citation command.

(And I shamelessly stole @lockstep's sample bib file, I pray for forgiveness).