I have read some reports that the 23, and maybe the 30-inch models have substantially brighter display panels recently, but there is no announcement of any kind about that, probably just an internal change and nothing more.
There was a model change in November 2005 with these displays, but no specification changes at that time. The Displays had to be sold as lead-free to comply with some European directive. That's the latest change of any kind with Apple's displays.