When Snow Leopard was announced, it was meant to refresh the all system layers when touching the interface. So far, all beta versions followed this schema, and if the engine is completely different, one can not spot the difference between Leopard and Snow Leopard.
However, it seems that Apple decided to introduce so changes in the GUI. IT does not seem to be radical modifications, but rather a new theme that will be unified to the system and all application running on it. So far we only know its name: "Marble".
This change is most likely restricted to the cosmetic point of view, and was a bit expected as Apple needs to justify visually some of the deep changes made internally in the system, the marketing team needs some material for the future campaign and does not want to only rely on the substantial performance gain brought by Snow Leopard.
