x11-libs/wxGTK-126.96.36.199 depends on app-admin/eselect-wxwidgets-0.7-r1, which is blocked by x11-libs/wxGTK-188.8.131.52-r2. I would assume that since wxGTK-2.6 & 2.8 are slotted, they are supposed to be able to coexist.
Steps to Reproduce:
1. emerge >=wxGTK-2.8-0
2. USE="wxwindows" emerge vlc (all currently depend on wxGTK-2.6)
[ebuild NS ] x11-libs/wxGTK-184.108.40.206-r1 USE="X opengl sdl unicode -debug -doc -gnome -joystick -odbc -pch" 0 kB
[ebuild R ] media-video/vlc-0.9.0_alpha20071022 USE="X .... wxwindows* ..." 0 kB
[blocks B ] <=x11-libs/wxGTK-220.127.116.11-r2 (is blocking app-admin/eselect-wxwidgets-0.7-r1)
Since the packages are slotted, I would assume they should coexist.
You need to unmask and upgrade 2.6 to 18.104.22.168-r3 which is not blocked. Mixing arch and ~arch can end up like this if you're not careful...