TikZ Styles that set Nodepart Styles

You can choose the font for a node part by using the normal TikZ key font=<font switch like \bfseries>. Note that the default Computer Modern font does not have a bold typewriter font. You can use the Courier font for that (see Using \ttfamily with \bfseries (or how to enable bold in fixed-width font)).

\documentclass{beamer}

\usepackage{tikz}
\usetikzlibrary{shapes}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}  




\usepackage{aeguill}
\newcommand*{\stereotype}[1]{
    \guillemotleft {#1}\guillemotright%
}                                      

\tikzset{class or interface/.style={%
        draw,%                    
        shape=rectangle split,%   
        rectangle split parts=3,% 
        rectangle split part align={center,left,left},%
        rectangle split part fill={#1!30!white,#1!20!white,#1!10!white},
        every two node part/.style={align=center, font=\ttfamily},
                     every three node part/.style={align=left, font=\ttfamily}, 
                     node distance=2
    },
         interface/.style={
        class or interface=green,
        every one node part/.style={align=center, font=\ttfamily\itshape}
    },                       
         concrete class/.style={
        class or interface=red,
        every one node part/.style={align=center, font=\ttfamily\bfseries}
    }
}                    
\newtheorem{exercise}{Exercise}                                             


\begin{document}
\renewcommand{\ttdefault}{pcr} % Choose typewriter font that supports bold
%
%
\begin{frame}[fragile]
   \frametitle{Class Diagram of the Observer Pattern}
   \begin{tikzpicture}]                              
      \path (0,0) node(Subject)[interface,anchor=east]               
                  {\nodepart{one}                                   
                     \stereotype{interface}\\                        
                     Subject                     
                   \nodepart{two}                                    
                   \nodepart{three}\tabular{@{}l}                    
                     attach( Observer o )\\                 
                     detach( Observer o )\\                 
                     notify( )                              
                   \endtabular}                                      
            (Subject.south)                                          
           +(+0.0,-1.5)node(ConcreteSubject)[concrete class,anchor=north]
                  {\nodepart{one}                                       
                     ConcreteSubject
                   \nodepart{two}                                        
                    Observer[] observers
                   \nodepart{three}\tabular{@{}l}                        
                     attach( Observer o )\\                     
                     detach( Observer o )\\                     
                     notify( )                     
                   \endtabular};
   \end{tikzpicture}
\end{frame}
%
%

\end{document}