Gamespot was lucky enough to get an interview with Lead Designer Jeffery Kaplan regarding the upcoming expansion, The Burning Crusade. Here is a clip from the Q&A:

GS: What sort of system will The Burning Crusade have for socketed items, and how does it compare to the item-socketing system in Diablo II: Lord of Destruction?

JK: The system will be fairly similar to that of Diablo II, along with some nuances that will make it unique to World of Warcraft. For example, we plan to introduce a new type of gem that grants bonuses that depend on the other types of gems your character is using. In addition, players will be able to overwrite gems, but at the cost of the gem they are replacing.

You can check out the whole interview here.