artsplugin-mpg123-3.5.0 doesn't have a ~ppc keyword and is the only artsplugin from kde3.5.x not to have one. I added the following line to my package.keywords: kde-base/artsplugin-mpg123 ~x86 and it did emerge normally. Please add the ~ppc keyword to the artsplugin-mpg123-3.5.0 ebuild.
Thanks for the report. Marked ~ppc.