Apple will introduce new versions of iOS and OS X at its annual developer's conference.
iOS 4.0.1 Being Released Today?
iPhoneHellas.gr claims that iOS 4.0.1 will be released today. They cite a "reliable" source for the information. The site, itself, has been proven to be reliable in the past with this kind of information. They previously pinpointed the exact release date of iPhone OS 2.2 over a week in advance.
iOS 4.0.1 would presumably include only bug fixes as well as the new signal strength indicators for the iPhone. The timing of the release the day before Apple's press conference is somewhat curious.