Highlighting particular bibliography entries

Here's an easy way:

\documentclass[12pt]{article}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{cite1,
  author = {Knuth, Donald},
  title = {{Title One}},
  journal = {Some Journal},
  year = {2005},
  volume = {1},
  pages = {1--42},
}

@book{cite2,
  author = {Knuth, Donald},
  title = {{A Way More Important Title}},
  publisher = {Oxford},
  year = {2009},
  address = {New York}
}
\end{filecontents}

\usepackage[T1]{fontenc}
\usepackage{biblatex-chicago}
\addbibresource{\jobname.bib}

%% Highlight entries
\usepackage[svgnames]{xcolor}
\DeclareBibliographyCategory{important}
% the colour definition
\colorlet{impentry}{Maroon}% let 'impentry' = Maroon

% Inform biblatex that all books in the 'important' category deserve
% special treatment, while all others do not
\AtEveryBibitem{%
  \ifcategory{important}%
    {\bfseries\color{impentry}}%
    {}%
  }

% Add books to 'important' category in preamble
\addtocategory{important}{%
 cite2,
}


\begin{document}

\nocite{*}
\printbibliography

\end{document}

A trickier way would be to use biber's \DecareSourcemap command (documented in biblatex manual) to essentially grep for a keyword in the keywords field and then act accordingly. One advantage of this method is that it means the 'initialization' is put in the .bib file rather that needing to declare new bibliography categories and so on. (You also get to re-write the bibliography output on the fly, so you could have only [say] an addendum field that is highlighted.)

There might also be reason to create other useful commands like an \addncite, which \cites the entry and adds it at the same time to this new 'important' category. Not sure what your overall needs are, however.