The following MWE displays the problem:
\documentclass{beamer}
\begin{document}
\begin{frame}\onslide<+->
\begin{tabular}{p{4cm}r}
Text\\Text & Text \\\hline
Text\\Text & Text \\\hline
Text\\Text & Text \\\hline
\onslide<+->Text & Why too high??\\
\end{tabular}
\end{frame}
\end{document}
When I remove the \onslide it looks correct. If I change the p{4cm} to say l, the this too works.
How can I get theis last line to be correctly aligned?