According to a twitter member, one of the staff at the Apple Store in Lakeside, UK has revealed that the update for iOS4 is being held back in an attempt to patch the loophole used by hackers to jailbreak iOS4.
Another reason for the delay could be, to see if hackers release the Spirit jailbreak for the iPhone 4 so that Apple can patch that as well and make the jailbreak redundant.
Guy in Apple Lakeside seems 2 think 4.0.1 is delayed as they r doing a fix 2 stop jailbreak that they have found. Dunno if true?
At this point this is just a rumor as Apple hasn't announced the exact date at on which the update will be released which leads people to think that they are probably delaying iOS 4.0.1/4.1 update.