[myTouch 3G, Magic General] sirocyl:
A bit of a personal opinion here:

"Dead" is when the phone doesn't respond to anything, at all.

The charging light does not come on, the power button does nothing, and there is not so much as a flicker in the screen. Also, it is completely inaccessible for hardware debugging (JTAG, serial, USB or otherwise). This usually happens from a hardware problem, or doing something stupid like zero'ing the IPL.

Examples:

- My brother dropped his Nokia N75 in water. It's dead.

- I dropped my Cingular 8125 (HTC Wizard) on the ground. It doesn't power on any more, and plugging it into the charger does nothing. Voltmeter shows a charge running through some areas of the device's motherboard once disassembled. It's dead.

Solutions: Replace, recycle or sell for parts.

"Bricked" is when a purely software problem causes the device to no longer respond.

This may happen on many levels, but usually results in the OS not booting, or booting and being completely unusable (e.g. stopped with kernel panic at OS splash). It may light up and/or show a splash screen, but it fails to enter the bootloader or recovery menus. Its recoverability may vary, but will usually involve a debugging tool and a computer, and likely a copmlete wipe of everything on the device. Even if it's heavily bricked, if it can be recovered via JTAG or similar, it's not "dead".

Examples:

- HTC Dream only goes as far as splash screen, and freezes there. Does not respond to ADB. Does not enter recovery, or bootloader, however fastboot responds.

- HTC Dream shows blank screen, but lights up as if it were powering on. Fastboot does not respond. I connect my HTC ExtUSB Serial dongle to it though, and I'm able to get a response to some commands.

- Western Digital MyBook World Edition NAS powers on, and spins up hard drive, but does not go any farther than that. Front lights do not light up, and no hard disk activity is heard. Networking is not yet enabled. Hard drive can, however, manually be connected to a PC with SATA, and the firmware image re-written to its partitions.

- XBOX (the original) was softmodded, but the dashboard.xbe (the OS shell) was overwritten. It now just stays at the Microsoft logo.

- iPhone was recently jailbroken and SIM unlocked, then Apple pushed an update on me, and my iPhone is in a state where it doesn't enter DFU/Recovery, and when it boots, it says "Emergency calls only."

Solutions: Be very careful not to further brick the device, but use any methods available. Be sure to check documentations, search forums, and then ask questions if unsure. Make sure you finish every process you start.

After this, might just be hardware "damage" (like a power button not working right) or software "errors", like the home screen failing to load because of "Process com.google.android.gapps quit unexpectedly." These are usually relatively easy to fix, or have fixed.

Again, this is just an opinion.

Back on topic though, I seriously wouldn't have probably thought of using fastboot while it's stuck at the G1 screen, unless mine were bricked. Kudos to you, and I hope this works on some other bricked G1's out there (At least ones with the Engineering/Hard/DangerSPL, the stock SPL doesn't support fastboot as far as I know.)