Tested on OpenBSD 4.5 i386
The default network card also doesn't work very well, set it to e1000 instead. The i8255i (fxp0) device seems to crash the vm when it tries to bring up the network.
Tested working ok with default settings on version 8 amd64
Works with the ne2k_pci driver with ACPI disabled, on single-core VMs. Untick the ACPI box on the VM management screen for this to work.
Current release (20090729) working ok on i386 with the ne2k_pci network card. Read the documentation before installing for the first time though,