This just happened to me when I loaded the newest 4.5 software on a friend's BB. I looked at his SIM card and it is a VERY old "VoiceStream" branded card. I downgraded him back to the latest 4.2 version and it worked again. The thing is, I tried his SIM in MY 4.5 version Pearl and same thing (after some delay). So, I am thinking that some older SIM cards are incompatible (capacity or speed issue?) with the 4.5 software. I suggested that he go into the T-Mobile store and grab a new SIM and then we will try it again.