Well, it looks like the dust has all but settled and U640 as a replacement for U320 SCSI is dead.
SATA over SCSI advocates are going to hate this: SAS (Serial Attached SCSI) controllers will be coming to market in the near future, delivering 3 GB/sec throughput, reason enough to abandon U640, and nearly doubling the max possible throughput of SATA (1.5 GB/sec). 6 GB/sec SAS will be the follow on (vs. SATA's follow on of 3.0 GB/sec).
There's nothing wrong with SATA technology for workstations and low end servers of course, but those of us who live or die by that last ounce of I/O subsystem performance understand clearly that it has no place in serious high load, high performance server applications. In these cases, only SAS will suffice.
Check the convenient comparison chart here.
Sorry SATA fans it was a nice try, but personally, I'm sticking to U320 until SAS is released. :-)