Doesn't look like there will likely be an XP Mode option available for Windows 8. At least not so far as Windows 8 Release Preview, it will not get past the "Activiation" portion of downloading and obtaining XP Mode / Virtual PC.
With their emphasis moving on toward VHDs and Hyper-V and their actual Virtual PC product seemingly halted at its' 2007 product version, I suspect that you will likely either need to employ one of these options or look towards something like
Virtual Box of
Link Removed for a solution. Unlike XP Mode, you will need your own licensed copy of XP in order to move forward with any of these.
Regards
Randy