Note: IntelliScreen conflicts with Cyntact in a non-obvious manner, and seems to cause problems with Cydia. I have logged hours of support time dealing with this problem today, making me very very sad. You need to turn on Cyntact under IntelliScreen's Performance tab due to the way IntelliScreen seems to damage MobileSubstrate. If this breaks Cydia, you will need to remove IntelliScreen.