I think it is a public beta, because I did download XP-64 build 3790 w/ SP1 about six months ago and burnt it onto a CD. Never used it, never bothered to install it. There was just no software for it.
I think it's better now. ATI and nVidia have probably released beta 64-bit drivers. It's posted in some thread here.
EDIT: Longhorn? We're talking about WinXP-64 bit. Unless the two OSs are unde rconcurrent development (which I dont think is).