After the last world update, revdep-rebuild emerges x11-libs/qt-demo-4.8.1-r1 which fails as: /usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lQtHelp collect2: ld returned 1 exit status furthermore, after emerging x11-libs/qt-assistant (which has been uninstalled authomaticaly during the last world update, i don't know why) x11-libs/qt-demo-4.8.1-r1 is emerging fine. So looks like x11-libs/qt-demo-4.8.1-r1 requires x11-libs/qt-assistant, but portage doesn't know this.
I can also reproduce this issue.
Fixed, thanks for reporting! 15 May 2012; Davide Pesavento <pesa@gentoo.org> qt-demo-4.8.1-r1.ebuild: x11-libs/qt-assistant is a hard dependency, because the qtdemo binary links against QtHelp (bug 416065).