HTC HD2 hasn’t even hit the markets yet, with some minor exceptions but we already have custom ROMs made by XDA Developers and HTCPedia. The only thing in their way now, besides the lack of the HD2, is a HardSPL. What’s a HardSPL?
HardSPL is a modified (hacked) bootloader which allows a much safer flashing process to take place since it reduces the chance of permenently bricking your device. It also removes the need to CID Unlock (vendor lock) is put on your device by the manufacturer to prevent installation of a ROM not released by them) the device since it ALWAYS reports SuperCID when flashing.
Think of the Bootloader as your Device’s BIOS. The bootloader is the first thing to load once you turn on the device, this does the job of booting up the device hardware and loading various items into memory before the ROM starts in the device. The bootloader is also responsible for enabling the flashing of new ROMs and other components such as the Radio and the Ext_ROM.
(Source: XDA Developers Wiki)