mplayer, as far as I can tell the only package that used the "3dnowex" flag, actually now has a USE flag "3dnowext". The ChangeLog for the mplayer package shows this was renamed for all mplayer versions on 16 April 2005 (mplayer-1.0_pre6-r2 was changed on 22 March). The flag was, however, not renamed in the use.mask files for non-x86 profiles, i.e. 'USE="3dnowext" emerge mplayer' actually tries to compile mplayer with 3dnowext support. In addition to renaming, some profiles - a simple grep in /usr/portage/profiles reveals at least alpha, bsd, mips, sparc and sparc64 - do not even mask "3dnowex".
Sorry, one sentence there is incomplete: ... 'USE="3dnowext" emerge mplayer' actually tries to compile mplayer with 3dnowext support ON MY SPARC64 MACHINE.
thanks for the notice
fixed now. the useflag mask is done by masking globally and unmasking when needed.