If you have been having any issues with the recent iOS 5.0.1 Untethered Jailbreak here is a possible fix for you. Saurik has just updated Mobile Substrate which should fix any issues with Winterboard, Activator, iBooks and more. This newest version won't even make you reboot your device after installation! You can find this under the changes tab after a refresh in Cydia. Mobile Substrate 0.9.3995 is a free update.