Fedora 17 S390x, a.k.a. IBM System z mainframe computers, isn't a primary architecture but is a secondary arch similar to the recent Fedora 17 ARM release.
Fedora 17 for s390x is improved over earlier Fedora releases with installer improvements, the Haskell compiler (GHC) now being provided for this secondary architecture, Eclipse is now available, and various other improvements. Fedora 17 is compatible with IBM System z9 mainframes and newer.
Additional information is available in the mailing list release announcement from Dan Horák out of Red Hat Czech and on the Fedora Wiki.