Positioning content at the top of a beamer slide (by default)

You could specify vertical top alignment globally by the t class option:

\documentclass[t]{beamer}

For single frames you could use the same option locally:

\begin{frame}[t]
...
\end{frame}

For those writing beamer themes: The class option t is stored in \beamer@centered. So \beamer@centeredfalse and beamer@centeredtrue will change the default vertical alignment for frames and columns.