Diskeeper 2008 defrags in real-time using idle resources, but it does not defrag continuously in the background 24x7. After it has finished defragging the drive completely the first time around, it usually runs for only a few minutes a day ...usually less than 5 mins on my drives to clear up the few remaining fragments.
Power loss during Diskeeper auto defrag is no more 'risky' than that during a manual defrag with the XP/Vista defragger.
The risk of data corruption due to power loss exists at all times, that's why important PCs always need to be connected to a UPS. If you pull the plug when Windows itself is writing to the MFT, there is a high chance that corruption will occur regardless of whether any defragger/program is in operation or not.