Cotulla announced this rather bizarre achievement via Twitter, with a stream of photos to prove he got the job done. Both the Metro environment and the plain old desktop run on the HD2, which presumably includes Office RT and the like.
In order to get all this to work, I've been told Cotulla had to write his own EFI bootloader - no small feat, and it demonstrates his dedication and skill (as if that was needed considering he already ported both Android and Windows Phone to the HD2). The fact that Windows RT runs on the HD2 at all is a small miracle - this is a phone from 2009, running on a single-core 1Ghz Snapdragon processor with 576 MB RAM.
We don't have any video material (yet), and the port hasn't been released to the public, so we don't yet know how well it performs. The legality of it all is of course shaky as well, but then, back in the Windows Mobile days Microsoft simply looked the other way and silently approved the ROM community. It would be a nice gesture if they continued to do so today.
I've always wanted an HD2, and this news really isn't helping.