Add Footer Text to All Slides in Beamer

Section 8.2.1 The Headline and Footline of the beameruserguide lists the predefined inserts that can be used to add information to the footline template. If you want to add some additional information (not covered by the listed inserts), you can use the text line option for the footline template.

A little example in which "some text" is added as well as the short name of the author centered and the page number flushed to the right (I used a \parbox with some negative vertical skip to shift the information a little bit upwards):

\documentclass{beamer}

\setbeamertemplate{footline}[text line]{%
  \parbox{\linewidth}{\vspace*{-8pt}some text\hfill\insertshortauthor\hfill\insertpagenumber}}
\setbeamertemplate{navigation symbols}{}

\author[JD]{John Doe}

\begin{document}

\begin{frame}
Test
\end{frame}

\end{document}

I can think of only one problem that the solution proposed by @GonzaloMedina has: when the slides number goes from 9 to 10 then there is a horizontal jump in the footline. The following solves the problem by putting each component of the footline in its own box:

\setbeamertemplate{footline}[text line]{%
  \parbox{0.8\linewidth}{
    \vspace*{-8pt}\insertshorttitle~(\insertshortauthor)
  }
  \hfill%
  \parbox{0.15\linewidth}{
    \vspace*{-8pt}\raggedleft\insertpagenumber
  }
}