Top alignment of cell content in tabularx

You can make the inner tabulars top align by using [t] It may just be an artifact of your small example but tabularx can do nothing to help here as the inner tabular is fixed to its natural width so can not reflow to whatever width the X column specifies.

\documentclass{article}
\usepackage{tabularx,booktabs} 

\begin{document}
\noindent
    \begin{tabularx}{\textwidth}{lX}
        \toprule
        Mittel    & technische Details\\
        \midrule
        %first row of tabularx
        Laptop      &
            \begin{tabular}[t]{ll}
            MacBook Pro & \\
            Baujahr 2010 & \\ 
            Prozessor & 2.4 GHz Intel Core 2 Duo \\
            Speicher & 4 GB 1067 MHz DDR3 \\
            Software & Mac OS X Lion 10.7.5 (11G63)
            \end{tabular}\\[3em]
        %second row of tabularx
        Programmierumgebung          &
            \begin{tabular}[t]{ll}
            Version & 8.0.4.0 \\
            Plattform & Mac OS x86 (32-bit, 64-bit Kernel)
            \end{tabular}\\
        \bottomrule
    \end{tabularx}

\end{document}

Why not use a single tabularx environment instead of one inside a tabular?

\begin{tabularx}{\textwidth}{llX}
    \toprule
    Mittel              & technische Details & \\
    \midrule
    Laptop              & MacBook Pro  & \\
                        & Baujahr 2010 & \\ 
                        & Prozessor    & 2.4 GHz Intel Core 2 Duo \\
                        & Speicher     & 4 GB 1067 MHz DDR3 \\
                        & Software     & Mac OS X Lion 10.7.5 (11G63) \\[3em]
    Programmierumgebung & Version      & 8.0.4.0 \\
                        & Plattform    & Mac OS x86 (32-bit, 64-bit Kernel)\\
    \bottomrule
\end{tabularx}

One can specify the vertical alignment -- top, middle (default), and bottom -- of a tabular environment via an optional argument. In the code below, I use [t] (for "top" alignment). Note that it's also necessary now to suppress explicitly, via a @{} directive, the whitespace that's otherwise inserted at the left edge of the cell when using the X column type for a cell that contains a tabular environment.

\documentclass{article}
\usepackage{tabularx,booktabs} 
\begin{document}
\noindent
  \begin{tabularx}{\textwidth}{lX}
    \toprule
    Mittel  & technische Details\\
    \midrule
    %first row of tabularx
    Laptop    &
      \begin{tabular}[t]{@{}ll} 
         % use [t] alignment specifier, and @{} to 
         % suppress extra whitespace at left edge
      MacBook Pro & \\
      Baujahr 2010 & \\ 
      Prozessor & 2.4 GHz Intel Core 2 Duo \\
      Speicher & 4 GB 1067 MHz DDR3 \\
      Software & Mac OS X Lion 10.7.5 (11G63)
      \end{tabular}\\[5em]
    %second row of tabularx
    Programmierumgebung      &
      \begin{tabular}[t]{@{}ll} 
      Version & 8.0.4.0 \\
      Plattform & Mac OS x86 (32-bit, 64-bit Kernel)
      \end{tabular}\\
    \bottomrule
  \end{tabularx}
\end{document}