There's a rumor going around that iOS 4.0.1 and/or iOS 4.1 have been delayed by Apple in order to combat potential Jailbreaks.
Firstly, Apple is secretive to the point that no one outside the iOS team and top brass likely knows when they're planning on releasing any iOS update, ever. There are plenty of bugs like the antenna reception, the proximity sensor, 3G speeds, the camera roll, etc. that need to be fixed before it can even be scheduled. Everyone else is guessing or passing on heresy, including Apple Store employees, carrier reps, and internet aficionados.
Secondly, Apple is almost as cyclical as they are secretive, so while we don't know the precise day iOS 4.0.1 or iOS 4.1 will be released, we know when iOS x.0.1 and iOS x.1 have been released in the past.