Yah.. they talked about this at an Xserve seminar I attended @ Apple's Cupertino campus. They made a big deal about the fact that you can take the hot-plug drives out of the xserve and add them to this raid box.
Just wished that they had one available. They also would not give any further info on when they expect to have them ready. The usual tight-lipped response typical of Apple.
Xserve is turning out to look like a very solid package for admins.