Had a rough scan through the tree; there are 73 packages that make direct use of mmx, sse*, 3dnow or altivec in IUSE (list below), excluding gcc itself. They also appear in the mythtv-plugin eclass so presumably some of media-plugins/myth* (11 packages) are also affected.
I don't know if altivec is as clearly determined from the target arch; perhaps the ppc people could chime in. Clearly most are in media-* categories, so that would be a good place to start :) app-crypt/johntheripper dev-lang/pike dev-lang/squeak dev-libs/DirectFB dev-libs/DirectFB-extra dev-php5/eaccelerator games-emulation/dgen-sdl games-emulation/visualboyadvance games-emulation/xmame games-emulation/xmess games-engines/exult media-gfx/gimp media-gfx/inkscape media-gfx/optipng media-libs/allegro media-libs/flac media-libs/gdk-pixbuf media-libs/imlib2 media-libs/libfame media-libs/libggi media-libs/libmovtar media-libs/libmpeg3 media-libs/libquicktime media-libs/mlt media-libs/sdl-gfx media-libs/smpeg media-libs/speex media-libs/xvid media-plugins/eq-xmms media-plugins/vdr-softdevice media-plugins/xmms-mpg123 media-sound/ardour media-sound/audacious media-sound/fluidsynth media-sound/jack-audio-connection-kit media-sound/mpg123 media-sound/noxmms media-sound/terminatorx media-sound/xmms media-sound/zynaddsubfx media-tv/mythtv media-tv/xawtv media-tv/xdtv media-video/avidemux media-video/cinelerra-cvs media-video/dirac media-video/dxr3player media-video/effectv media-video/fame media-video/ffmpeg media-video/gephex media-video/mjpegtools media-video/mpeg4ip media-video/mplayer media-video/ogle media-video/recmpeg media-video/transcode media-video/vlc media-video/xmovie net-irc/xchat net-irc/xchat-gnome net-misc/asterisk sci-chemistry/gromacs sci-libs/acml sci-libs/fftw x11-base/xorg-x11 x11-libs/evas x11-libs/libast x11-misc/rss-glx x11-terms/eterm x11-themes/polymer x11-wm/afterstep x11-wm/metisse -- Kevin F. Quinn
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