From Linux Reviews.org:
"Some packages are masked , meaning they won't install unless they are specifically demanded.
A package may be masked because it fails to compile or has serious bugs. It may also simply be masked because the latest version to brand new and therefore not tested enough to be included in the main portage tree.
Masked does not mean dangerous or unstable. Usually the latest stable release is what is used by default, while the development releases are masked.
A package can be masked for one, a few or all architectures (x86, ppc, sparc, sparc64, or alpha).
To install a masked package on x86:
ACCEPT_KEYWORDS="~x86" emerge mozilla-firebird-cvs
It is quite possible to set ACCEPT_KEYWORDS="~x86" in /etc/make.conf. This is generally a bad idea, specially if you have no idea what you are doing.
To check for the latest masked version of a package:
ACCEPT_KEYWORDS="~x86" emerge -pv gimp
Some packages do not want to be installed even if you are using ACCEPT_KEYWORDS="~x86". These packages are randomly specified by the nice gentoo development theme and configured in:
If you are wondering if there is a later version of a package than what is available as a masked package in portage, try:
grep gimp /usr/portage/profiles/package.mask
If this shows you a line like..
.. then edit package.mask and add a # in front of that line, or:
mv package.mask package.mask.tmp
sed s/gimp/#gimp/g > package.mask "
In other words, your error message is simply telling you that it is unwise to update that particular package, so I recommend that you don't do it. I would just run "emerge -uD world" and let portage get on with it.