As of May 2012, the instructions have changed. goldfish (the emulator) now has its own git repository at: https://android.googlesource.com/kernel/goldfish.git
To use this repository, you have to have the commit ID for a known good kernel. (By default, after an initial clone, the repository is bare.) You are supposed to be able to find out the commit id by looking at the last git log message for the file: prebuilts/qemu-kernel/arm/kernel-qemu-armv7 (the prebuilt kernel for the emulator).
You can use this in the goldfish repository, by doing 'git checkout -b mybranch <commit id>'
Then do 'make goldfish_defconfig', and then 'make'.
To use your kernel, put it in $ANDROID_PRODUCT_OUT/kernel_qemu (usually, this is
out/target/product/generic/kernel_qemu), and the emulator will find it automatically.