Preview at WWDC likely in June, followed by September launch.
Apple Releases Firmware Update to Address Screen Flickering on 27-Inch iMacs
About 27-inch iMac Graphics Firmware Update 1.0
Updates the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or display flickering.
Apple's new 27-inch iMac models have been plagued with reports of issues since their launch beginning in late October. Early reports of Flash performance issues were fixed with the release of Mac OS X 10.6.2, but more recent reports have revealed machines being delivered in a non-functional state or with cracked screens.