According to section 18.2.1 of the beamer manual, a default font size of 8pt is already "way too small". beamer uses the extsizes packages for non-standard font sizes, and extsizes doesn't offer anything smaller than 8pt. If you really must fit more text on each frame, I suggest to enlarge beamer's paper size from 128mm*96mm to, say, 140mm*105mm.
\documentclass[8pt]{beamer}
\geometry{paperwidth=140mm,paperheight=105mm}
\usepackage{lipsum}
\begin{document}
\begin{frame}
\lipsum[1]
\end{frame}
\end{document}

EDIT: Another option is to load the scrextend package (part of KOMA-Script) which allows to choose arbitrary font sizes like 7.5pt -- be sure to use a vector font like Latin Modern.
\documentclass{beamer}
\usepackage{lmodern}
\usepackage{scrextend}
\changefontsizes{7.5pt}
\usepackage{lipsum}
\begin{document}
\begin{frame}
\lipsum[1]
\end{frame}
\end{document}

extsizesandKOMA-Scriptclasses, but not aboutscrextend. – lockstep May 21 '12 at 17:05