If you're using the "rotate cell" button in the table toolbar, the problem is
that the multirow command is being nested inside the sideways environment; you
need the nesting in the opposite order. Try deselecting the rotation button in
the toolbar and use something like the following inside the cell (I'll use
tildes to denote start/end of ERT):
~\multirow{2}{*}{\begin{sideways}~ your text ~\end{sideways}}~
You'll need to add \usepackage{rotating} to the preamble (or trick LyX into
doing it by rotating an empty cell in the table), and you may need to add a
little space after your text (say, a hard space or two) to get perfect
centering.
/Paul
