First, the limitation is on the boot
device, not the boot
partition. The boot device cannot be larger than 2TB, regardless of the partitions that are on it.
The reason for that is that the PC standard BIOS cannot address a device that is larger than 2TB, because the PC BIOS is designed as a 32-bit system. The BIOS can load the OS files on a device up to the 2^32 block, after that, the BIOS has no way to address blocks that are further out on the device. So if ntkernel or ntldr (for Windows) resides on a block past 2^32 (which would be legal on a device that's bigger than 2TB), the BIOS cannot load that file, thus the machine won't boot.
Now, you might think that if the boot partition resides completely within the first 2TB that you'd be OK, but that's not the case. When the BIOS goes to access the device, it has to know its size, and if the size is larger than 2TB, it can't address it.
The Itanium machines have an EFI BIOS, a new and completely different BIOS that has the ability to address blocks up to 2^64 on the boot device. Thus, it can boot to a device larger than 2TB.
After everything was cleared up in this thread, it was stated that a >2TB boot device is not supported on normal PC hardware, regardless of operating system. (See #2 in my first post in the thread).