How to set the maths font in Beamer when using XeLaTeX?

As you found out yourself, \usefonttheme{professionalfonts} is needed to be able to use opentype fonts with beamer.

However, I want to add that you probably should use unicode-math together with xelatex to change the math-fonts.

\documentclass[serif]{beamer}

\usefonttheme{professionalfonts}
\usepackage{fontspec}
\setromanfont{Linux Libertine O}

\usepackage{mathtools}

\usepackage{unicode-math}
% check name of font for your system, this works on Ubuntu
\setmathfont{TeX Gyre Pagella Math}

\begin{document}

\begin{frame}{Unicode-Examples}
  Some unicode in formulas:
  \begin{equation}
    α² + β² = γ²
  \end{equation}
  And a fraction: ½.
\end{frame}
\end{document}

For some reason the comments containing the answer have disappeared, but I hope it is not impolite to answer my own question on the basis of those comments.

The solution is to use \usefonttheme{professionalfonts} in the preamble.

The minimum working example is now:

\documentclass[serif]{beamer}
\usepackage[utopia]{mathdesign}
\usepackage[no-math]{fontspec}
\setmainfont{Liberation Serif}
\usefonttheme{professionalfonts}% SOLUTION
%
\begin{document}

\begin{frame}
\frametitle{Minimum working example}
\begin{itemize}
    \item normal text set in Liberation Serif and maths \textbf{also,} instead of Adobe Utopia as intended
        \[v_x(t) := \lim\limits_{\Delta t \rightarrow 0} \frac{\Delta x}{\Delta t} =: \frac{\mathrm{d}x}{\mathrm{d}t}\]
\end{itemize}
\end{frame}
\end{document}

Producing

(having corrected the physics mistake above as well).