The following are the ADP1 / G1 button combinations for booting in various modes.

Regular mode	Power up via "Call End" button
Fastboot mode 1	Power up via "Camera" + "Call End" button. Wait until Android on skateboard image appears with text "SERIAL0", and press Back button. It'll enter FASTBOOT mode.
Fastboot mode 2	Power up via  "Call End" + "Back" button takes you directly to fastboot mode
Recovery mode	Power up via "Home" + "Call End" buttons. In recovery mode, Alt + L - toggle log text display Alt + s - apply sdcard:update.zip Alt + W - wipe data/factory reset
Safe mode	Power on via "Call End" + "Menu" button

The 3-button salute (Call + Menu + Hangup) causes a reboot in any of the above modes.