Milestone's kernel is based on Google's Eclair (Android 2.0) release which is available here.
Motorola has released the sources to its modifications of that Android kernel and other GPL code included with the Milestone. There are several Motorola patch versions available. The patch version is part of the Build Number that identifies the firmware version:
* Latin American Milestone 01.03.0
* Latin American Milestone 01.03.1 (this version is identical to the Latin American 01.03.0, and doesn't seem to exist in real Milestone phones)
These patches could be used to create our own boot images, if it were not for the fact that the Milestone's bootloader checks the image for a signature by Motorola.
Droid's source release link can be found at Motorola's open source project.
It is interesting that Droid's kernel is merely the Android Open Source Project 2.6.29-Eclair of kernel/omap.git while Milestone has a separate source repository in Motorola. The difference in both's kernel source may reveals what needs to be fixed in order to use Droid's bootstrap element on Milestone, in case both device shares the same boot ROM public key.
Latest OMAP3 Integration linux tree here http://dev.omapzoom.org/?p=integration/kernel-omap3.git
Latest OMAP3 Integration Android tree here (2.6.32 branch is active) http://android.git.kernel.org/?p=kernel/omap.git;a=shortlog;h=refs/heads/android-omap-2.6.32
Latest OMAP Syslink tree here:
Kernel side http://dev.omapzoom.org/?p=tisyslink/kernel-syslink.git;a=summary
Userspace side http://dev.omapzoom.org/?p=tisyslink/userspace-syslink.git;a=summary
Bootloaders from TI:
U-Boot : http://dev.omapzoom.org/?p=bootloader/u-boot.git;a=summary
X-Loader : http://dev.omapzoom.org/?p=bootloader/x-loader.git;a=summary