Apple just released an update for iMacs sporting either a ATI Radeon HD 2600 Pro or a ATI Radeon HD 2400 XT graphics card. According to Apple, the update will "improve system stability" by installing a firmware upgrade on the graphics cards. This does affect the newly released iMacs.
After this update is installed, you should see ROM version: 113-B2250L-259. You can download the update by opening Software Update (Apple menu > Software Update) on the affected system, or by downloading the installer package directly from Apple.
Apple also notes: To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/iMac ATI Radeon HD Graphics Firmware Update.app). The updater will launch automatically when the Installer closes.