I have a Matrix modchip currently.
I had my X-ecutor modded Xbox online with Live beta since September. The week before launch, I modded my friend's Xbox by:
I gave him my X-ecutor modded Xbox mobo and he kept his DVD ROM and his HD.
I got his virgin mobo, which I modded with Matrix, and then I kept my DVD ROM and HD.
So, I was able to connect to Live with both systems when I was done. But, the following Tuesday, Live won't connect anymore on my Matrix system. Since I read about the EEPROM Magic tutorial, I decided to do it since I still had his c:\backup on my PC. I redid my EEPROM and I tried to connect to Live with my "new" EEPROM (which was actually my old EEPROM from my beta days) and I still can't connect.
So, there you go.
EDIT: Hmmm, that might be a little confusing. So, I will add this. This is about the EEPROM. I used the EEPROM form beta from September to Monday, November 11. My Xbox was always modded when I used Live beta.
Since I was modding the weekend of November 9, I connected to Live with my friend's EEPROM, which was on a Matrix modded system (which became mine).