Booting straight from the .iso, in VirtualBox, same configuration as with 2.5, and I still get the "ACPI: Unable to load the System Description Tables" message. System comes right up, though, and seems to be fully functional, unlike RC1, that took nearly 5 minutes just to get past the ACPI message.
Is there any additional information you would like about the configuration, dmesg, etc that would be of assistance? 2.6 is the first release (RC or otherwise) to produce an error on this box, I'd like to help fix that if I can.