Running WHS Beta 2. After an uneventful installation (except for multiple restarts that was a bit annoying) on an Opteron 165, 1GB RAM, 160GB HD, Asus A8V SE MB, 1-Sony DVD ROM, and 1 Sony CD-RW, I had to manually install multiple hardware devices.
Those devices included: Radeon 1300 video card, Marvell Yukon ethernet adapter, Realtek sound device, Asus ACPI, and Via chipset drivers. All took going to the vendor websites, grabbing the XP/Server 2003 drivers and applying to the system. No complaints by any of the drivers about unknown Windows versions.
Once applied everything has been stable with 3 user accounts in addition to the admin and guest (disabled) accounts. Haven't tried automated backups yet.
Bottom line, box has been operating at 100% stress level (running Seti@Home on both cores to load the system) for 48 hours (as of now) and all is well. Next step is to start pounding the "share" features of what appears to be a stable box.