Digitimes reports that VIA has drivers for release candidate 2 of Windows XP 64. I've not found them for download, but they may be included with RC2 itself.
VIA?s complete driver lineup for 64-bit operating systems includes AMD K8T/K8M-series for desktop PCs, K8N-series for notebooks, Pentium 4-compatible PT880 Pro and PT894 series and embedded applications including the CN400 that supports Windows XP.