This seems like such a simple topic - but its complicated by dirty little details you probably didn't realize.
Question: When I start up my xbox 360 - My Xbox Live profile does not sign in automatically, how do I change this?
Answer: Everyone says just find it in your profile's settings. But in the NXE dashboard, you've searched and searched and searched and can't find it. Why? Here's the dirty little detail. Didn't you know.. not all your settings are IN the dashboard? The Guide button on your remote (the big X button) when pressed brings up a quick menu, that gives you quick access to various things on your xbox. But did you know.. there is configuration that is ONLY accessible via this menu? Yup.. and that's where auto sign-in is. When you hit the Guide button, goto the settings blade. Then click Profile. This menu looks identical to when you edit your gamer profile in the dashboard... except... now there is an auto sign-in option! Set it here and go home happy.
Maybe sometime soon Microsoft will fix this lapse of reason in their NXE experience. The identical pages are NOT identical in the guide vs dashboard. Poor UI Microsoft.. Poor UI.