tenor ~ # cat /etc/portage/package.mask 
>=media-video/mplayer-1.0_rc1-r2
tenor ~ # emerge -pvt mplayer

These are the packages that would be merged, in reverse order:

Calculating dependencies /
!!! All ebuilds that could satisfy "mplayer" have been masked.
!!! One of the following masked packages is required to complete your request:
- media-video/mplayer-1.0_rc1-r2 (masked by: package.mask)
- media-video/mplayer-1.0_rc1-r1 (masked by: package.mask)
# Steve Dibb <beandog@gentoo.org> (26 Feb 2007)
# Security bug 159727

- media-video/mplayer-1.0_rc2_pre20070321-r4 (masked by: package.mask)
- media-video/mplayer-1.0_rc1 (masked by: package.mask)
- media-video/mplayer-1.0_pre8-r1 (masked by: package.mask)

For more information, see MASKED PACKAGES section in the emerge man page or 
refer to the Gentoo Handbook.

tenor ~ #