Set math and text fonts independently in Beamer (using PDFLaTeX)

\documentclass{beamer}  
\usepackage[T1]{fontenc}
\usefonttheme[onlymath]{serif}
\renewcommand\sfdefault{cmbr}
\begin{document}
\begin{frame}
Some text, $x^2 + y^2 = r^2$
\end{frame}
\end{document}

or

\documentclass[professionalfonts]{beamer}  
\usepackage[T1]{fontenc}
\usepackage{cmbright}
\usefonttheme[onlymath]{serif}
\begin{document}
    \begin{frame}
    Some text, $x^2 + y^2 = r^2$
\end{frame}
\end{document}