I am using the Eye-Fi 8GB and new 16GB Pro X2 cards in my D800. They fixed the problem they were having with the D800 and Direct Mode, with a firmware upgrade. Eye-Fi now says the D800 is compatible with their cards.
They work fine on my D800. However, they are slooooooooow with a D800 file due to the size. JPEGS aren't too bad, it takes about 2 or 3 minutes to transfer ONE image. Forget RAW and on TIFF, just say NO!
For file transfer, stick to your USB cable.
============================================== Darrell Young (DigitalDarrell) www.pictureandpen.com "Better too many words than not enough understanding." ==============================================