At this point, the only differences that can be discerned are in operating system (open source with propietory parts versus closed source and licensing model with ODM modifications key to differentation), and availability. But its pretty clear that they are going for similar markets.

HTC might get a few more looks because they are aiming more for those who are iPhone-fans, and then also grab developers who already are familiar with the MS development environment. Devices like this and a few other WM devices are going to blurr the line between smartphone and tabletPC/UMPC all the more though, and that will work out great for end-users.

Maemo will have to do some work on the UI side if they see this and other devices as competitors.

EDIT: the HTC HD will have a 562Mhz CPU, not way faster, but faster than what we have seen from IT devices so far. The key is to understand that with mobile devices, fast CPUs are nothing if the system cannot take advanage of it. WinMobile needs a fast CPU for many tasks because of how its built. If HTC is keeping the ability of the process to scale, then that max speed rating is just max, not consistent.