In addition to new, denser platters, the 7200.9 sports all the features detailed in the recently drafted Serial ATA 2.5 spec. The Serial ATA 2.5 spec includes a number of required and optional features, and the 7200.9 supports them all, including 300MB/sec transfer rates, Native Command Queuing, hot plugging, and staggered spin-up. The 7200.9 series can also auto-negotiate a host transfer rate with a Serial ATA controller, so you don't have to worry about switching the drive between 150MB/sec and 300MB/sec modes manually.
120GB, 160GB, and 500GB Barracuda 7200.9 drives have been shipping for more than a week, and they've already arrived at Newegg. Seagate expects the rest of the 7200.9's available capacities to become available over the next two months.