Occasionally RPM isn't as smart as it thinks it is, you could try ignoring it's dependencies with the "--nodeps" flag, like:
rpm -i mplayer-1.0.rpm --nodeps
It might not work, but no harm done if it doesn't, cause you can always "rpm -e mplayer" later to get rid of it.
You almost certainly do -not- want to do that.
Questions for the OP:
* What mplayer are you trying to install? (stock? livna? freshrpms?)
* Did it actually install or not? (You said you got the dependency issues -after- installing, which doesn't sound right)
* What's "rpm -q mplayer libpostproc" say?
General Suggestion:
Use one of the friendlier rpm frontends like up2date, yum, or apt-get, as they handle dependencies much better and at least tend to provide more useful error messages. They will require some minor configuration, see older posts in this list or the unofficial faq for more information.
