G1 without internet

[imtheone]

Ok, I am getting a mytouch and my friend wants my g1. He has unlimited texting, but he doesnt want internet because he has WiFi at his house and he doesnt think he will ever use internet while he is out and about. So, I put his SIM card in and it connected to the internet. Is that because my stuff is still on it? Or once I format it and use it with his stuff instead of mine it will say that it cant connect right? My G1 is rooted and everything, and before I had internet on it it just said that it couldnt contact the server, once it is setup for him it will do the same right?