In a blog posting by Mac-centric sales outfit Other World Computing (OWC), the company mentions that the for the new 2011 iMacs has unlocked SATA III capabilities in the systems, allowing for up to 6Gb/sec data throughput on the internal hard drive bays. The past iMac and MacBook Pro models have shipped with SATA II drive controllers that handle up to 3Gb/sec throughput, but the new systems apparently use updated controllers.
Apple's iMac technical specifications Web page for the new systems just states the hard drives use "Serial ATA" connections, but does not indicate the generation of SATA controllers used. After updating the systems with the latest firmware release, however, the systems are showing capabilities up to 6Gb/sec throughput for the hard drives, though the optical drive apparently is still at 3Gb/sec.
OWC is currently benchmarking the systems with the new update, which could show the previous 3Gb/sec notation might have been a false labeling, or that it actually did increase the speed of drive bus in the new systems. If the latter is in fact the case, then users will likely see a marked increase in speed, especially if they have upgraded their systems with SATA III-capable SSD drives.
Update, 11:30 a.m. PDT: The new iMac line apparently uses the latest Z68 chipset from Intel, which offers a number of improvements and features including support for hybrid SSD/HDD drives and 6Gb/sec SATA support for up to two drives according to Z68 specifications, which Apple's update has unlocked. The chipset is apparently supposed to launch on May 8, but Apple apparently was able to get them in the new iMac systems early.