VirtualBox 3.1.2r56127 (win64, win32, linuxMint 5 x86 - & Debian 5.0 x64 hosts)
I've just been booting with the raw .iso, which produces the ACPI errors.
I rebuilt the .iso using the non-UPX'd bzImage, and it works fine.
Is there any more information about my setup that would be useful? Debug logs, dmesg output, the VirtualBox .xml file, etc?