The following are the ADP1 / G1 button combinations for booting in various modes.
Regular mode: Power button
Fastboot mode: Power on with Camera + Power button. Wait until Android on skateboard image appears with text "SERIAL0", and press Back button. It'll enter FASTBOOT mode.
Alternatively, Power + Back button takes you directly to fastboot mode
Recovery mode: Power on with Home + Power button.
In recovery mode,
Alt + L - toggle log text display
Alt + s - apply sdcard:update.zip
Alt + W - wipe data/factory reset
Safe mode: Power + Menu button
The 3-button salute (Call + Menu + Hangup) causes a reboot in any of the above modes.