Pre-orders and previews begin April 10, launches April 24.
Apple Preparing for Mac OS X 10.5.1
No details on the revision are yet available, but Apple has traditionally pushed out updates quickly after initial release. Apple released the first .1 update to Mac OS X 10.4 (Tiger) less than three weeks after its initial release.
Meanwhile, we are still waiting for the Mac OS X 10.4.11 update which has been expected any day now. Apple even references the unreleased update in a number of technotes.