It looks like another company has entered the 3TB hard drive arena. According to this post over at Akiba PC Hotline!, a 3TB Hitachi Deskstar 7K3000 hard drive is selling in Japan for approximately 22,000 yen, or $264 USD.
While there has been no announcement from Hitachi, I was able to find a product page for the new drive. This page reveals the following specs: a 7,200-RPM spindle speed, a 6Gbps Serial ATA interface, 64MB of cache, a 3.5" form factor, and rated noise output of 29 dB.
As we discussed in our recent review of Western Digital's Caviar Green 3TB, the 3TB capacity in itself induces some limitations. In a nutshell, if you want to boot off this drive, you may need one of the rare motherboards with a Unified Extensible Firmware Interface instead of a BIOS, and you'll also have to be running a 64-bit version of either Windows 7 or Vista.