RPM is not all
High RPM gets you low latency, but it doesn't guarantee high transfer rates. Take a look at the Samsung Spinpoint F1 HDs - at 7200RPM they beat WD raptors at 10k without a problem, all while delivering higher capacity, consuming far less power and making less noise at less cost. They do it by increasing data density on the platter (334Gb/platter), so while the disks are spinning slower, more data is passing the head per second.
@Dr Mouse: "Wouldn't surprise me if they were, or at least will be soon, making controllers chips that do both SATA and SAS"
This is already true. Pretty much all SAS controllers can also accept SATA drives. The reverse is not true. It's nice because you can mix fast SAS drives with big SATA drives. Along with the advent of 15krpm 2.5" SAS drives (cute but expensive), you can now get 1U rack cases (See supermicro) that host 8 hot swap 2.5" drives.