After an emerge -e world I noticed all my WX appz (audadcity, vlc for example) stop working. Searching the web I found this bug report https://bugs.launchpad.net/mandriva/+source/vlc/+bug/120278 So using G_SLICE=always-malloc aoss audacity they work again. This problem should affects both 2.6 and 2.8 version but a patch seems to exist (see the link)
x11-libs/wxGTK-2.6.4.0 works great ;)