Mac Rumors and Computerworld are both reporting that Apple appears to have increased the quality of displays used in its unibody 13-inch MacBook notebooks sometime towards the end of April.
The Computerworld article which cites discussions and photos posted on the Mac Rumors forums points towards a change in model numbers, photos show some differences but one user even offers a description of the difference.
I personally see a difference in person when I use them both, but others may not. The newer one seems to have much deeper blacks and better contrast. I'm not sure its as good as the screens in the uMBP's but IMHO its definately a "step-up" from the original uMB screens.
It is thought that the new displays are very similar if not identical to those used in the MacBook Air.