We've been stuck at 2TB for what seems like forever, and there's good reason for that. Drive partitions larger than 2.19TB create a unique problem for PCs, and trying to boot from them requires a mish mash of technologies, including the use of a GPT partition, a modern 64-bit OS (Vista or Windows 7), and a motherboard equipped with an EFI BIOS.
Despite all this, Western Digital has gone ahead and begun shipping a 3TB hard drive in Caviar Green trim, which qualifies as the largest capacity internal SATA drive around. The drive utilizes four 750GB platters as well as Western Digital's Advanced Format technology, which you can read more about here.
To sidestep the issue of integrating large capacity hard drives into your system, Western Digital also bundles an Advanced Host Controller (AHCI)-compliant Host Bus Adapter with its 3TB (and 2.5TB) hard drives, which makes it easier for OSes to locate and use a known driver with correct support for large capacity drives. In other words, you can actually boot from the thing without any crazy voodoo.
The 3TB drive is available now for for $240, while the 2.5TB drive sells for $190.