The good folks over in marketing made a sneak announcement on December 29th regarding the Disk Library for mainframe (DLm). Sneak because, lets be honest, how many of us had reading press releases as a high priority on December 29th?
In any event, there is some good stuff here. To revisit the core differentiator of the EMC DLm: this is a virtual tape library for mainframes that is not a disk cache. That is a significantly different architecture from either competitive offering from Sun or IBM--both of which are "merely" a small to medium size disk cache in front of a bunch of physical tape. Expensive disk. Expensive tape.
So EMC's approach is a little different: take the tape out of the equation. And a good chunk of the cost too. The DLm is far more than "just" a disk cache: by virtue of capacity and capability it enables you to store data more or less permanently on disk. Not to say that you will have no tape at the end of the day, but you will have a lot less than with other approaches.