Usually, whenever a new KDE release is published, Gentoo users can update already the same day, as suddenly a complete and polished set of ebuilds appears in the portage tree. (Stay tuned on upcoming wednesday for KDE 4.8.0, it's shaping up very nicely!) How is this possible? Well... let me explain.
If you're a stable version user, you may have never heard of so-called live ebuilds. This is a special variant, usually denoted by a version number ending in 9999, that does not rely on a source tarball. Instead, it contains a URL of a revsion control system (say on anongit.kde.org). When you emerge such a version of a package, the sources of the specified branch are checked out or updated to the newest upstream state, and that is used for building the installation package. Obviously this is not for everyone; depending how well upstream structures commits, things may not build for a while, contain fresh bugs, ... Also, reporting bugs from live versions on Gentoo bugzilla is discouraged as most of the times we can't do anything about it (do it only if you are sure it's a problem with the ebuilds, not with the source). If you're running live, you should be willing to hack yourself and work with upstream.
However, many of the Gentoo KDE team members run these live ebuilds, partly the current bugfix branch (i.e. KDE/4.8), partly even git master. They continuously keep the live ebuilds in the Gentoo KDE overlay updated to the newest state of the source. When a release is made, the corresponding live ebuilds of this branch are copied to the version ebuilds. For example, the KDE/4.8 branch live ebuilds have the version number 188.8.131.5299 (i.e.
kde-base/kdelibs-184.108.40.20699), so when the pre-release tarballs for KDE 4.8.0 were released to the packagers a few days ago, we only had to copy all 220.127.116.1199 ebuilds to 4.8.0 and immediately had a working set for testing. Most problems at that point are only caused by changes in tarball packaging. As distribution packagers get the pre-release tarballs (that still may change due to last-minute bugfixes) a week before the official release date, these can easily be fixed in time.
This also means that KDE maintenance in Gentoo is really a team effort. Whoever moves a released version to the main portage tree and/or commits bugfixes there builds on all the work that the team has done in the overlay in the meantime. Cheers!