In know virtualization is not one of the primary targets of OpenBSD. However, in case someone is interested, here's a dmesg of 4.4-current booting bsd.rd on latest XenServer 5 (Express, with Intel VT). As you can see, there is no harddisk detected.
I am ready to help testing if a developer wants to look at it. Cheers, Stephan OpenBSD 4.4-current (RAMDISK_CD) #883: Wed Sep 17 13:17:23 MDT 2008 [EMAIL PROTECTED]:/usr/src/sys/arch/i386/compile/RAMDISK_CD cpu0: Intel(R) Xeon(R) CPU E5410 @ 2.33GHz ("GenuineIntel" 686-class) 2.33 GHz cpu0: FPU,V86,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,CFLUSH,MMX,FXSR,SSE,SSE2,SSE3,CX16 real mem = 267976704 (255MB) avail mem = 252547072 (240MB) mainbus0 at root bios0 at mainbus0: AT/286+ BIOS, date 06/23/99, BIOS32 rev. 0 @ 0xfa4e0, SMBIOS rev. 2.4 @ 0xe901f (11 entries) bios0: vendor Xen version "3.2.1" date 06/23/1999 bios0: Xen HVM domU apm0 at bios0: Power Management spec V1.2 acpi at bios0 function 0x0 not configured pcibios0 at bios0: rev 2.1 @ 0xf0000/0x0 pcibios0: PCI IRQ Routing Table rev 1.0 @ 0xfa780/128 (6 entries) pcibios0: PCI Interrupt Router at 000:01:0 ("Intel 82371SB ISA" rev 0x00) pcibios0: PCI bus #0 is the last bus bios0: ROM list: 0xc0000/0x8c00 cpu0 at mainbus0 pci0 at mainbus0 bus 0: configuration mode 1 (bios) pchb0 at pci0 dev 0 function 0 "Intel 82441FX" rev 0x02 pcib0 at pci0 dev 1 function 0 "Intel 82371SB ISA" rev 0x00 pciide0 at pci0 dev 1 function 1 "Intel 82371SB IDE" rev 0x00: DMA, channel 0 wired to compatibility, channel 1 wired to compatibility wd0 at pciide0 channel 0 drive 0: <QEMU HARDDISK> wd0: 16-sector PIO, LBA48, 5120MB, 10485760 sectors wd0(pciide0:0:0): using PIO mode 0, DMA mode 2 atapiscsi0 at pciide0 channel 1 drive 1 scsibus0 at atapiscsi0: 2 targets, initiator 7 cd0 at scsibus0 targ 0 lun 0: <QEMU, QEMU CD-ROM, 0.8.> ATAPI 5/cdrom removable cd0(pciide0:1:1): using PIO mode 0, DMA mode 2 "Intel 82371AB Power" rev 0x01 at pci0 dev 1 function 2 not configured uhci0 at pci0 dev 1 function 3 "Intel 82371SB USB" rev 0x01: irq 5 vga1 at pci0 dev 2 function 0 "Cirrus Logic CL-GD5446" rev 0x00 wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation) unknown vendor 0x5853 product 0x0001 (class mass storage subclass SCSI, rev 0x01) at pci0 dev 3 function 0 not configured re0 at pci0 dev 4 function 0 "Realtek 8139" rev 0x20: RTL8139C+ (0x7480), irq 5, address c2:37:9e:eb:1b:19 rlphy0 at re0 phy 0: RTL internal PHY isa0 at pcib0 isadma0 at isa0 com0 at isa0 port 0x3f8/8 irq 4: ns16450, no fifo pckbc0 at isa0 port 0x60/5 pckbd0 at pckbc0 (kbd slot) pckbc0: using irq 1 for kbd slot wskbd0 at pckbd0: console keyboard, using wsdisplay0 npx0 at isa0 port 0xf0/16: reported by CPUID; using exception 16 usb0 at uhci0: USB revision 1.0 uhub0 at usb0 "Intel UHCI root hub" rev 1.00/1.00 addr 1 biomask ffed netmask ffed ttymask ffff rd0: fixed, 3800 blocks uhidev0 at uhub0 port 2 configuration 1 interface 0 "QEMU 0.8.2 QEMU USB Tablet" rev 0.10/0.00 addr 2 uhidev0: iclass 3/1 uhid at uhidev0 not configured softraid0 at root root on rd0a swap on rd0b dump on rd0b