Update #4: The b4 version of redsn0w incorporates the 5.0.1 fix for iBooks, and also for sporadic problems with launchctl. Thanks to @xvolks for merging the iBooks (sandbox) fix from @comex’s github into the overall corona untether from @pod2g! As usual, you can choose to install the fix either by re-running redsn0w over your existing jailbreak (de-select Cydia if you do that), or by installing the corona package from Cydia (it’s the same set of files no matter which way you choose).
TIP: If auto-detection fails and redsn0w tells you no identifying data was found, you can always pre-select the appropriate 5.0.1 IPSW using “Extras->Select IPSW”.
Here are the redsn0w download links:
redsn0w 0.9.10b4 for OS X
redsn0w 0.9.10b4 for Windows (be sure to run in Administrator mode)
Enjoy these fixes in your untethered jailbroken iPad and iPhone 4.