tokyoahead awstats # uname -a
Linux tokyoahead 2.6.16-gentoo-r7 #1 SMP Tue May 23 15:37:14 HKT 2006 i686 Intel(R) Celeron(R) CPU 1.70GHz GenuineIntel GNU/Linux

tokyoahead awstats # emerge -Dup world

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

Calculating world dependencies... done!
[ebuild  N    ] dev-util/unifdef-1.20
[ebuild     U ] sys-apps/busybox-1.5.1 [1.4.2]
[ebuild     U ] sys-apps/hdparm-7.5 [7.3]
[ebuild     U ] sys-kernel/linux-headers-2.6.21 [2.6.17-r2]
[ebuild     U ] sys-apps/man-pages-2.62 [2.60]
[ebuild     U ] dev-perl/Net-Daemon-0.41 [0.39]
[ebuild     U ] dev-perl/MIME-Types-1.20 [1.18]
[ebuild     U ] dev-perl/HTML-Parser-3.56 [3.55]
[ebuild     U ] net-misc/curl-7.15.5-r1 [7.15.1-r1]
[ebuild     U ] dev-perl/PlRPC-0.2019 [0.2018]
[ebuild     U ] dev-db/mysql-5.0.44 [5.0.42]
[ebuild     U ] media-libs/gd-2.0.35 [2.0.34]
[ebuild     U ] dev-perl/DBD-mysql-4.00.5 [4.00.4-r1]
[ebuild     U ] dev-libs/apr-util-0.9.12-r1 [0.9.12]
[ebuild     U ] sys-apps/file-4.21-r1 [4.21]
[ebuild     U ] net-ftp/ftpbase-0.01 [0.00]
[ebuild  NS   ] sys-kernel/gentoo-sources-2.6.21-r4  USE="-build -symlink"
[ebuild     U ] dev-libs/glib-2.12.12 [2.12.11]
[ebuild     U ] app-arch/mt-st-0.9b [0.7-r1]
[ebuild     U ] dev-perl/Crypt-SSLeay-0.55 [0.54]
[ebuild     U ] app-arch/rar-3.7.0 [3.7.0_beta1]
[blocks B     ] sys-kernel/linux-headers (is blocking sys-kernel/linux-headers-2.6.17-r2, sys-kernel/linux-headers-2.6.21)


tokyoahead awstats # emerge -Du world
Calculating world dependencies... done!

!!! Error: the sys-kernel/linux-headers package conflicts with another package;
!!!        the two packages cannot be installed on the same system together.
!!!        Please use 'emerge --pretend' to determine blockers.

For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked