/etc/make.conf has USE="-kde -arts -qt -qtmt -gnome"

emerge --pretend x11-wm/xfce produces

Calculating dependencies ...done!
[ebuild N ] media-libs/lcms-1.08 to /
[ebuild N ] media-libs/libmng-1.0.3 to /
[ebuild N ] x11-libs/qt-3.0.3.1 to /
[ebuild N ] kde-base/arts-1.0.0 to /
[ebuild R ] x11-wm/xfce-3.8.16 to /

but if I do this

export USE="-kde -arts -qt -qtmt -gnome"
emerge --pretend x11-wm/xfce

I get

Calculating dependencies ...done!
[ebuild R ] x11-wm/xfce-3.8.16 to /

Also Weija Yang reported that USE="-alsa" in make.conf then emerge kdemultimedia fails but USE="-alsa" emerge kdemultemedia works.
Can you repaste this as plain text in the comments box?
What version of Portage are you using?
Pretty sure this is fixed in Portage 1.9.5. Closing.