Recovery Mode on Motorola Milestone
What is it for?
A recovery method is required whenever we need to try new kernel versions on the Milestone. Without a recovery method, any unsuccessful test would be the last one (the phone would be "bricked"). The standard recovery method for Android is the recovery mode.
How to access it
You can access the recovery mode on the milestone by shutting down the device and start it by pressing simultaneously Power+CameraButton (on 90.78 bootloader it is Power+X button).
After you get the Warning image you can access the menu by pressing simultaneously CameraButton+VolumeUp
How to run ADB in Recovery Mode: ADBrecovery
The daemon adbd runs unprivileged in the Milestone because property ro.secure is set to 1 in default.prop (you can use the getprop command to check this). But we can use the su command to run adbd as root anyway.
proof of concept of this idea evolved into first version of a recovery patch for Milestone which in turn evolved into current ADBrecovery version, now incorporating a port of nandroid and being to backup and restore several key Milestone partitions.
Developed by Skrilax_CZ.
Credit for mankind (from alldroid) for the CustomUpdate, on which this recovery is based,
credit for poseidon (also from alldroid) for ADBRecovery.
Current version 1.14 [06/11/2010]
Open Recovery is a fully customised recovery using the payload hack to restart the stock recovery into itself. Supports easy rooting and taking backups from the menu and easily extendable. The page is here: Open Recovery
There's an interaction pathway between the OS and the recovery stage, using commands written into the misc partition. See the source code of the GPL motobox command by Motorola. See  how the bootloader (mbm or lbl) communicate with the OS in order to update the radio firmware and the boot image itself.