As of last night I became the now "frustrated" owner of the new slim model not having done my research. I came across an interesting issue that was bugging me. It comes to the auto-sign in issue where a disc can be left in the tray. However on my old Falcon model (360 Classic/Arcade) with a 120gb hard drive I did not have this issue. Last night I updated, data transfered, transferred my licenses over and then left my Defiance disc in till today. I went to lunch, came back, turned on my XBox and low and behold I could not connect to XBox Live via Auto-Sign In. Instead I had to click the "connect to XBox Live" black image with the XBox Live banner on it, vwalah, connected. After doing some research I came across fellow gamers that have had this same issue where they would not auto-sign into XBox Live, but only their profiles.
Annoying as it is I have discovered you can't leave discs in currently when you get ready to shut down your XBox. Something about the disc drive being active is not allowing the XBox to sign in automatically to XBox Live. So I took my disc out, shut off my XBox 360 slim 250gb model (Forza 4/Skyrim bundle). Turned it on and now it's auto-connecting. I will be thoroughly testing this to see if it is a fix, if it is. Microsoft, there's something wrong, please take a look at it. Many people are in having a very rough time over this. I'm saddened I had to dig for almost 7 hours to find a "fix". For official source of figuring this out, read the forum link I discovered. Link: http://forums.xbox.com/xbox_forums/xbox_support/f/9/t/393545.aspx