The Android source code includes at least two ARM sub-architectures in their kernel tree. The one actually used on the G1 hardware is derived from the Qualcomm MSM machine type, at least part of which is in the mainline. Then there is “goldfish,” which appears to be just a pseudo machine used only for the emulator. While I haven’t had a lot of success with the MSM stuff booting on the phone so far, building the emulator kernel is not too hard once you know where to look:

# get the MSM specific tree.
# You don't really need it but might as
# well grab both up front into the same git repo.
$ git clone git://android.git.kernel.org/kernel/msm.git msm
$ cd msm

# also grab the common one for emulator target and check it out
$ git remote add common git://android.git.kernel.org/kernel/common.git
$ git fetch common
$ git checkout -b goldfish common/android-goldfish-2.6.27

$ cat <<_EOM >make-arm.sh
#! /bin/bash

# I use the codesourcery toolchain
PATH="$XGCC/arm-2008q3/bin/:$PATH" make ARCH=arm CROSS_COMPILE=arm-none-eabi- "$@"

$ chmod 755 make-arm.sh
$ ./make-arm.sh goldfish_defconfig
$ ./make-arm.sh

Now the emulator can be run with something like:

$ export ANDROID_PRODUCT_OUT=`/bin/pwd`/out/target/product/dream
$ emulator -verbose -show-kernel -kernel msm/arch/arm/boot/zImage