Upgrading anything in Gentoo is quite different better architected than the rest of Linux world. One of those is the concept of a profile:

A profile is a set of configuration files, stored in a subdirectory of /usr/portage/profiles, that describe things such as the ebuilds that are considered system packages, the default USE flags, the default mapping for virtual packages, and the architecture on which the system is running.

The profile in use is determined by the symbolic link /etc/make.profile, which points to a subdirectory of /usr/portage/profiles which holds the profile files. For instance, the default x86 2005.0 profile can be found at /usr/portage/profiles/default-linux/x86/2005.0. The files in the parent directories are part of the profile as well (and are therefore shared by different subprofiles). This is why we call these cascaded profiles.

This profile architecture made it easy to upgrade packages while keeping my 2.4 kernel in place. There was no compelling reason to upgrade the kernel - until a few weeks ago. The upgrade was incredibly simple, but like many, many, others I missed an important step: properly handling the migration from devfs to udev.

Be sure to account for the new device model in udev. Specifically you will need to change /etc/X11/XF86Config to use /dev/psaux instead of /dev/mouse. The complete Gentoo Linux 2.6 migration guide does point this out - but in a rare case of poor documentation, it is not called out properly.