Hard drive manufacturers have been slow to ship a 3TB product, at least partly because it's a pain to create a bootable operating system partition on a volume larger than 2.19TB. The Master Boot Record (MBR), which tells your computer the location of each sector, can only address 2^32 (4,294,967,296) sectors, which amounts to 2.19TB when using the typical 512-byte sector format. Things get hairy beyond that capacity.
To create a boot volume larger than 2.19TB, you need a 64-bit OS, support for GPT partitions (which can address 2^64 sectors), and a motherboard with UEFI instead of the standard BIOS. Since most current systems don't yet support UEFI standards, WD has included a known-compatible AHCI-compliant PCI-Express Host Bus Adapter (HBA) card as a workaround. HBA adapter or not, Windows XP 32-bit and 64-bit users are out of luck.
Nonetheless, it's obvious that the 3TB Green is mostly intended to be used for secondary bulk storage, and as Storage Review notes, the rules are slightly different in a non-boot scenario. The average PC user doesn't need a 3TB boot volume anyway and would be better served by something smaller and quicker. Western Digital has priced the 3TB WD30EZRSDTL at $239 (HBA card included), while a 2.5TB iteration (WD25EZRSDTL) is $189.