So… qt-*-4.6.0_rc1 is now in the portage tree, masked (since its not a final release) and you can begin testing your shiny Qt applications with it.
But there’s a catch.
4.6.0_rc1 is not binary compatible with 4.6.0_beta1. 
This means that if you’re upgrading from 4.6.0_beta1 you have to rebuild every single app depending on qt-*, or they won’t start at all. This obviously includes all of KDE 4.
You can get portage to do that by using a command like this:
*update* it seems some shells didn’t like the old command, so I’ve updated it:
emerge -av1 $(for i in $(qlist -IC x11-libs/qt-); do equery -q d $i | grep -v 'x11-libs/qt-' | sed "s/^/=/"; done)
Note that this command assumes your system is up-to-date. If some installed packages don’t have ebuilds available for their version anymore, you’ll have to resolve that manually (probably by removing them and rerunning the command).
You’ll find equery in app-portage/gentoolkit and qlist in app-portage/portage-utils.
You don’t need to do this if you’re upgrading from Qt 4.5.3 (although you should). You’ll also be safe if you upgrade from 4.5.3 to 4.6.0 final when the time comes.
Portage will warn you about this when you upgrade qt-core to 4.6.0_rc1:
* Messages for package x11-libs/qt-core-4.6.0_rc1:
* Binary compatibility broke between 4.6.0_beta1 and 4.6.0_rc1.
* If you are upgrading from 4.6.0_beta1, you’ll have to
* re-emerge everything that depends on Qt.
* Use the following command:
* emerge -av1 $(for pkg in $(equery -q d \
* $(qlist -IC x11-libs/qt-) | grep -v “x11-libs/qt-” |
* sort -u); do echo “=$pkg”; done)
* YOU’VE BEEN WARNED
But users (my user side as well :p) tend to ignore ewarns from time to time