Gentoo Linux: "Package NOT merged due to file collisions" problem and solution

When trying to get the latest sys-libs/timezone-data on a somewhat old system, I got the following error:

[list of nearly all files the package was about to install]
 - Package 'sys-libs/timezone-data-2008i' NOT merged due to file
 - collisions. If necessary, refer to your elog messages for the whole
 - content of the above message.

According to equery, it looks like those files were installed by glibc:

# equery b /usr/share/zoneinfo/CET
sys-libs/glibc-2.3.4.20041102-r1 (/usr/share/zoneinfo/CET)

So I tried

# COLLISION_IGNORE="/usr/share/zoneinfo" emerge -av timezone-data

which showed less errors but still gave me:

 - sys-libs/glibc-2.3.4.20041102-r1
 -      /usr/bin/tzselect
 -      /usr/sbin/zdump
 -      /usr/sbin/zic

I ended up with

# COLLISION_IGNORE="/usr" emerge -av timezone-data

which worked well.

I looked at the ebuilds and saw that newer glibc packages depend on sys-libs/timezone-data, so upgrading glibc should solve the problem too. I've submitted bug 258802 since I didn't find the info anywhere else.