Why bother with LIPO? Add the other architecture to your build flags (i.e. specify both -arch i386 AND -arch ppc) and see if that works first. If it doesn't, the LIPO approach WILL work.
I tried that at first. And when I compile for -arch ppc only, it has an error during the compile.
edit
I compile with export CFLAGS="-arch ppc"; ./configure --prefix=/ffmpeg/lame --build=ppc --disable-nasm
and the error I get is
/bin/sh ../libtool --tag=CC --mode=link gcc -Wall -arch ppc -fno-common -static -o lame main.o console.o get_audio.o lametime.o parse.o portableio.o timestatus.o brhist.o ../libmp3lame/libmp3lame.la -lncurses -lm
mkdir .libs
gcc -Wall -arch ppc -fno-common -o lame main.o console.o get_audio.o lametime.o parse.o portableio.o timestatus.o brhist.o ../libmp3lame/.libs/libmp3lame.a -lncurses -lm
Undefined symbols:
"_init_xrpow_core_sse", referenced from:
_init_xrpow_core_sse$non_lazy_ptr in libmp3lame.a(quantize.o)
ld: symbol(s) not found
collect2: ld returned 1 exit status
make[2]: *** [lame] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2