Okay, so as soon as I typed that I realized that I just put a new 20GB hard drive in my Xbox (I bought a new one, which works ... and am preparing this one for sale to a friend who doesn't care about XBL). So, I did the Slayer's install quick, and booted the MS Dash.
XBL still will not connect. Now, this is with a brand new HDD, and my EEPROM flashed back to the original EEPROM for this Xbox (banned from XBL, but still, it doesn't even connect far enough to tell me it's banned). I didn't even get into account recovery, selected the Xbox Live tab from the MS Dash, and was prompted with "Unable to establish Network Connection". Then, into the Network Troubleshooter and same old error.
My XYZ codes, since I don't know what they mean:
X:0000-F001
Y:00C2-03E0
Z:0000-0000
No AFFF or FFFF as was suggested above. I'm betting that somewhere along the line, while I was plugging/unplugging the network cable with all the rest of my equipment on (router, cable modem, etc.) ... something got zapped. Who knows. I didn't realize this many people were having trouble with it, though ... makes me wary about putting 4980/1 on my new Xbox.