This is one possible where each column is defined by P style (thus array package is required) in which 2cm is assumed; then the width of the 3 columns is determined by dimexpr where 6cm comes from 3 x 2cm.
Update: The OP tries m{1cm}, so the 6cm should be changed to 3cm.

Code
\documentclass[border=5pt]{standalone}%[a4paper,11pt]{report}
\usepackage[english]{babel}
\usepackage{booktabs,array,multirow}
\newcolumntype{M}{>{\centering\arraybackslash}m{1cm}}
\newcolumntype{P}{>{\raggedright\arraybackslash}p{2cm}}
\begin{document}
\begin{tabular}{|cc|*{3}{M}|}
\hline
\multicolumn{2}{|c|}{\multirow{2}{*} {Parameters}} & \multicolumn{3}{p{\dimexpr 3cm+2\arrayrulewidth+4\tabcolsep\relax}|}{This is some long text about precision and recall}\\
\hline
n & p &S& P & R\\
\hline
3000 & 0.5 & 3000 & 0.01 & 1.00\\
3000 & 0.1 & 3000 & 0.01 & 1.00\\
3000 & 0.008 & 30 & 1.00 & 1.00\\
\hline
\end{tabular}
\end{document}
So if 3cm is assumed for S,P,R column, then 6cm should be changed to 9cm.

Code
\documentclass[border=5pt]{standalone}%[a4paper,11pt]{report}
\usepackage[english]{babel}
\usepackage{booktabs,array}
\newcolumntype{P}{>{\raggedright\arraybackslash}p{2cm}}
\begin{document}
\begin{tabular}{|cc|*{3}{P}|}
\hline
\multicolumn{2}{|c|}{Parameters} & \multicolumn{3}{p{\dimexpr 6cm+2\arrayrulewidth+4\tabcolsep\relax}|}{This is some long text about precision and recall}\\
\hline
n & p &S& P & R\\
\hline
3000 & 0.5 & 3000 & 0.01 & 1.00\\
3000 & 0.1 & 3000 & 0.01 & 1.00\\
3000 & 0.008 & 30 & 1.00 & 1.00\\
\hline
\end{tabular}
\end{document}
m{1cm}? I just tried and the result is as expected. Did you define\newcolumntype{M}{>{\centering\arraybackslash}m{1cm}}and change the 6cm to 3cm? – Jesse May 04 '14 at 08:01