Apple has always said that the new version of OS X dubbed "Snow Leopard" would be about performance gains and not new visuals. With that in mind it has been circulating the web in resounding fashion that Apple will release a new theme for OS X when the product is launched.
A mockup (below) gives a possible rendering of what the new theme may look like. There is not much else to go on but when a story gets this big it generally means there is some credibility behind the source.
Keep your eyes peeled as it is expected that this theme will be unveiled anytime between now and the upcoming WWDC. Was Apple trying to drop a big surprise by saying that the focus was under the hood rather than on appearance, we don't know yet, but hopefully soon the truth will come into the light.