We've known for awhile that Leopard will support ZFS, so that wasn't too shocking. This is the first anyone has heard about ZFS being the default filesystem for Leopard though. Why is this a big deal? Well, ZFS is a filesystem that Sun built from the groundup to address modern needs at the filesystem level. ZFS is a 128bit filesystem meaning it can theoretically hold up to 16 exabytes worth of information across multiple devices (an exabyte, in case you don't know, is about 1,073,741,824 gigabytes so the ZFS can hold 17,179,869,184 GB of data though that would be many, many harddrives). Wikipedia has a full explanation of ZFS, which highlights all the cool features.
I suppose we'll all find out on Monday.