The OS X 10.8.3 update just hit, and had this gem:

Boot Camp support for Macs with a 3TB hard drive

We’ll be digging into it bit more, but it appears that the support is not due to EFI support for booting Windows, but rather some fancy use of core storage in OS X. The Windows partition still exists below the 2.2 TB boundary, and core storage combines the partition before and after the Windows partition to be a single volume. Very similar approach to the one outline on the site, but has the additional benefit of being able to merge the partitions without rebooting (though we haven’t confirmed this yet).

Here is the support doc that lists the feature: