The notion of a “tethered” jailbreak

The notion of a “tethered” jailbreak is pretty new to many people, so here’s a quick rundown on what to expect:

1. If you’re on an iPhone3G, old-bootrom iPhone3GS, or non-MC ipt2g, life is easy. redsn0w installed an untethered jailbreak and so nothing below applies.
2. “Tethered” does not mean you cannot boot at all without PC/Mac assistance.  If you have not installed any tweaks that hook into important programs like SpringBoard or CommCenter, your device will actually boot.  However, jailbreak programs like Cydia won’t work (and Cydia may still have a white icon).  Also, certain built-in apps that had to be moved by Cydia will fail (Safari being the most noticeable example).
3. If you’ve installed MobileSubstrate tweaks that hook into SpringBoard or other important programs, your boot will actually fail (you’ll get stuck at the Apple logo).  You need to use redsn0w to “Just boot tethered right now”.