Update - the latest revision of this chip is the dual-core PowerPC 970FX, which is a dual-core version of the 64-bit PowerPC 970, AKA G5. This chip hasn't found its way into any Apple products as yet, but several of the newer IBM P-series workstations and servers are based on these chips. Available operating systems include Linux, AIX and most likely NetBSD. (Rest assured that if NetBSD doesn't yet run on the 970FX, it will soon!)
The stalwart PPC7400 has received a major speed boost, too, running at a maximum of 1.66 GHz. It forms the core of the iBook, Powerbook and Macintosh Mini lines, which are currently not based on 970s for reasons of cost or power dissipation.
64-bit chips of the PowerPC series, like the UltraSPARC and AMD64 series, can run unmodified 32-bit binaries with only a minor performance hit (and, paradoxically, a performance gain in some odd edge cases, because 64-bit values are larger and take longer to copy about in RAM). Currently, Mac OS X, the best-known OS for PPC, is 32-bit (but can run 64-bit applications). The next OS X release will have native 64-bit capability on PPC970 machines. Linux, NetBSD and AIX already can run in native 64-bit mode.