How to add text to the corner of a Beamer slide?

The textpos package could be an option; in the following example, the command\FrameText puts its argument ragged right, at the bottom right of the frame; change the settings according to your needs:

\documentclass{beamer}
\usepackage[absolute,overlay]{textpos}
\usetheme{Warsaw}
\useoutertheme{infolines}
\setbeamertemplate{navigation symbols}{}
\newcommand\FrameText[1]{%
  \begin{textblock*}{\paperwidth}(0pt,\textheight)
    \raggedright #1\hspace{.5em}
  \end{textblock*}}
\begin{document}
\begin{frame}{Test frame}
\FrameText{Some text}
\end{frame}
\begin{frame}{Test frame}
\FrameText{Some text for frame two}
\end{frame}
\end{document}