Here is my working Configure for cross-compiling FFmpeg on
iOS 6 the arch is ARMv7

NOTE: You must have to have gas-preprocessor.pl inside /usr/local/bin/ please
do not continue until you have gas-preprocessor.pl on
your bin directory

• Download FFmpeg 1.0 "Angel" from here

• Unzip it and place it somewhere i.e. your Desktop folder

• Open terminal and browse to unzipped
FFmpeg folder

• Copy and paste the following command, (be patient will take a while)

./configure --disable-doc --disable-ffmpeg --disable-ffplay --disable-ffserver --enable-cross-compile --arch=arm --target-os=darwin --cc=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc --as='gas-preprocessor/gas-preprocessor.pl
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc' --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk --cpu=cortex-a8 --extra-cflags='-arch armv7' --extra-ldflags='-arch
armv7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk' --enable-pic --enable-decoder=rawvideo --disable-asm

• Now type the following command on terminal make (wait
  a little more)

• Once it has finished now type on terminal sudo
make install (wait again)

• Go to /usr/local/lib to
  find your freshly baked armv7 libs

• Enjoy!

  Added Support for armv7s

  This armv7s configure is totally untested and i dont really know if this would work, i don't own an iPhone 5 so we need someone to test the final armv7s
  libs

  ./configure --disable-doc --disable-ffmpeg --disable-ffplay --disable-ffserver --enable-cross-compile --arch=arm --target-os=darwin --cc=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc --as='gas-preprocessor/gas-preprocessor.pl
  /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc' --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk --cpu=cortex-a8 --extra-cflags='-arch armv7s' --extra-ldflags='-arch
  armv7s -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk' --enable-pic --enable-decoder=rawvideo --disable-asm