jbglaw@elle:/tmp/beamertest$ cat systematik_2009.tex
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{bibunits}
\usepackage{hyperref}
\usepackage[german]{babel}
\title{foo}
\author{me}
\begin{document}
\defaultbibliography{systematik_2009}
\defaultbibliographystyle{plainurl}

\frame{ \titlepage }
\frame{
        \frametitle{Seitentitel}
        \begin{bibunit} Text \cite{blub123} {\tiny \putbib} \end{bibunit}
}
\end{document}


jbglaw@elle:/tmp/beamertest$ cat systematik_2009.bib 
@article{blub123,
        author =        {Xxxx Xxxx},
        title =         {tiiitel},
        journal =       {Tolles Ding},
        number =        {1},
        pages =         {60-75},
        publisher =     {.p.},
        volume =        {20},
        year =          {2012},
        doi =           {xxxx},
        url =           {uuuu}
}


$ pdflatex s*tex
$ bibtex bu1
$ pdflatex s*tex              *booom*