Preview at WWDC likely in June, followed by September launch.
iPhoto 4.0.2 Now Available [Updated]
iPhoto 4.0.2 addresses minor issues with Smart Albums and European books, and provides notification when new versions of iPhoto are available.
The update can also be downloaded here (requires iPhoto 4.0.1 in iLife 04).
[Update] Apple has pulled the update, both from Software Update and from downloads. So far, no reason has been given.