I've set CFLAGS="-march=athlon -O3 -funroll-loops -pipe" in /etc/make.conf (GCC 3.1 profile), yet gnome-applets-2.0.1 compiles with "-O2 -march=i686 -pipe".
It's even worse: It uses "-mcpu=i686", not "-march=i686".
Forget it. It looks like I've updated Portage with CONFIG_PROTECT="" set, so it overwrote my CFLAGS in /etc/make.conf. My apologies.