As the subject says.

If I have AMD-V enabled in VirtualBox, I get a Kernel Panic. If I disable AMD-V, it hangs after ACPI setup and CPU detection. Last message is SMP motherboard not detected. I have tried all combinations of ACPI settings available in the VirtualBox GUI.

I also tried other images that I had availabe (SMS Live, Xen Server, DSL). Only DSL works. All others hang regardless of AMD-V setting. So it's a mixed bag and I don't really know what's wrong.

Update: found a solution by enabling workaround described in the VirtualBox manual (section 11.3.2):

Most Linux-based guests will fail with AMD Phenoms or Barcelona-level Opterons due to a bug in the Linux kernel. Enable the I/O-APIC to work around the problem (see Section 3.7.1.2, “"Advanced" tab”)