Apple just pushed first build of Mac OS X 10.5.5 to developers with build number 9F5. Dont have details right now; but will post soon.
EDIT: It comes with warning not to use the software with Macs that contain integrated graphics chips. "Do not install on the following machines: MacBook Air, MacBook, Mac Mini, iMac (with Intel integrated graphics)".
EDIT: It comes with warning not to use the software with Macs that contain integrated graphics chips. "Do not install on the following machines: MacBook Air, MacBook, Mac Mini, iMac (with Intel integrated graphics)".