Official Dev-Team just posted a Update #2 More Jailbreak Warnigs Nov. 24

[Why and How to Fix] White Cydia Icon and Safari Crashing When Using Redsn0w 0.9.6b4 to Jailbreak iOS 4.2.1

Update #2:  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”.

Remember, @comex and others are working on a way to untether the 4.2.1 jailbreak.  Meanwhile, the above 3 points hopefully will make it all seem less confusing :)
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 You can avoid the reasons above to enjoy a jailbreak free of bugs (i.e. White Cydia Icon and/or Safari Crashing), but I highly recommend to wait for the comex untethered jailbreak on 4.2.1 which expected to be very professional (still no ETA).

`` by darksman;

its very simple

for all new iphones 4 and 3gs after JB is done U will see white cydia icon dnt worry power off the phone and open redsn0w again point to fw 4.2.1 again and tick just boot tethered right now
after the process is done you will be able to open cydia but remember u will need to do this every time the phone reboots

cheers ``