Higher Max. areal density (Gbits/sq. inch) 11.0 (for the GXP75) vs. 15.3 (for the GXP60). This should translate into faster sustained throughput (40.8MB/s vs. 37.3MB/s for the GXP75). 10% faster throughput is worth something. It's also extremely quiet - IBM claims that it's the quietest 7200rpm hard drive. It is definitely an improvement over the GXP75, but I'm not sure that, if I needed a hard drive now, I would wait for the GXP60.
According to this IBM press release
here
It should be shipping now. So, presumably you should be able to buy them in volume later in Jan.