Different line spacings for main text and footnotes in beamer

It's a design feature. T. Tantau defends the opinion that there should not be a footnote on a slide. Stuff that is not inline with his view is usually difficult to do in beamer and many macros are overwritten accordingly. To be honest I agree with him after suffering from undescribable pain in many conferences. Funny thing is that most people are driving beamer to act like PowerPoint which is interesting to see.

\documentclass{beamer}
\usepackage{setspace}

\setbeamertemplate{footnote}%
{%
  \parindent 1em\noindent%
  \raggedright\setstretch{1}%
  \hbox to 1.8em{\hfil\insertfootnotemark}\insertfootnotetext\par%
}

\begin{document}
\setstretch{1.5}
\begin{frame}

foo\footnote[frame]{bar}\par
foobar

\vspace{2cm}

\begin{minipage}{.5\textwidth}
foo\footnote{bar}

bar\footnote[frame]{Nam dui ligula, fringilla a, euismod sodales, sollicitudin vel, 
wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis vitae, 
ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat ligula 
aliquet magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hendrerit mollis.}
\end{minipage}

\end{frame}
\end{document}