Building Buildroot 2011.02 with Crosstool-NG 1.10.0, using gcc 4.5.2 and uClibc 0.9.31, I get this error:

AS arm/simple_idct_neon.o
arm/mpegvideo_arm.c:33:1: error: size of array 'x_H263_AIC' is negative
make[3]: *** [arm/mpegvideo_arm.o] Error 1
make[3]: *** Waiting for unfinished jobs....
CC matroska.o
CC matroskadec.o
CC md5proto.o
make[3]: Leaving directory `/home/mjhammel/src/ximba/bb/bld/buildroot-2011.02/output/build/mplayer-32726/ffmpeg/libavcodec'

Doing some googling it appears that there is a problem with a defined value:
http://comments.gmane.org/gmane.comp.video.mplayer.user/64844
http://permalink.gmane.org/gmane.comp.video.mplayer.user/64851 - this is the most telling post
http://code.google.com/p/chromium/issues/detail?id=59123

Chromium has patches that might apply and appear to be generic (not ARM specific):
http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/ffmpeg/source/patched-ffmpeg-mt/libavcodec/arm/asm-offsets.h?r1=62568&r2=62567&pathrev=62568&diff_format=s
http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/ffmpeg/README.chromium?r1=62568&r2=62567&pathrev=62568&diff_format=c
http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/ffmpeg/source/patched-ffmpeg-mt/libavcodec/arm/asm-offsets.h?r1=62568&r2=62567&pathrev=62568&diff_format=c

I have not tried these because it turns out I don't need mplayer after all since this patch only makes mplayer use NEON with the processor but does not utilize the DSP on the BeagleBoard.  So I just disabled mplayer from the build.