You don't need JTAG for flashing, you only need it for developing the bootloader itself (which isn't necesary as the serial port and ROM should work the same way as on iPAQ afaik). The bootloader exists and it's developers seem quite capable. But the hard part would be hacking the linux kernel to support the radio part (signal monitoring, call handling etc.) and modifying it to be realtime-capable (because of the radio). Then, phone integration to the environment (userland), but that wil be quite easy...
Personaly, I doubt there will be a linux port ever unless HTC provides some real support (do they? :)), but it would be soooo cooooooooool :))