I've stated this in other posts, but I'll share for this one as well:
My NTSC/U console is banned. I modded my firmware long before an ethernet cable was plugged into my 360. It has an EEPROM chip. I updated to the hitachi 2.4 f/w BEFORE I applied the spring update.
My friend's NTSC/U console is banned. I modded his firmware AFTER he was on LIVE to 1.x, and haven't updated since. His NTSC/J console is NOT banned; modded firmware the day get got it to 1.x, and it has the spring update and same (no jitter) firmware.
My other friend's NTSC/U AND NTSC/J consoles are banned. He was on LIVE on the US one before modding it, but I don't believe he was on LIVE on the J one before modding. Either way, since I had mine modded for several months before even having a gamertag, I think it's safe to throw that theory out of the window.
I'd also like to add that I didn't connect to XBL the day of the bans. The next day I unplugged my DVD drive and booted my console to see if it would connect to LIVE and found I was banned (with no DVD). Of course, reconnecting the DVD did nothing other than allow me to run discs again.