If you followed the method described in wiki, you don't have sliderotate installed.
You will have to install the kernel an xserver-xomap again as instructed in the wiki or
uninstall becomeroot and click this:

http://mikie.dy.fi/maemo/sliderotate-diablo.install

and follow the instructions.

becomeroot conflicts with rootsh, which is a dependency in sliderotate.
Both provide basically the same effects, but since there is no need for an extra repository for rootsh I prefer that over becomeroot.