I don't have the SB800 (and I have to look at the manual when I put my flash on my film Nikon), so I'll just keeping asking questions to see if we can figure out the answer together, or help the next poster figure it out for you.
Did you buy the flash new or used? Did it ever show distance info? Have you used i-TTL flash before, or is this new for you also? (We all had our first times with everything.)
Do you have the manual, and have you looked it up in the manual?
Here's a pdf copy of the manual:
http://www.nikonusa.com/pdf/manuals/...hts/SB-800.pdf Aha! According to page 122 of the manual under Troubleshooting:
The flash shooting distance range does not appear: Answer: The flash head is adjusted to other than the horizontal/front or down -7° position.
Does changing the flash head orientation work for you?
http://www.kenrockwell.com/nikon/ittlslave.htm Ken Rockwell also explains how to use your D80's on board flash to fire your SB800 wirelessly -- it might be worth bookmarking for future use.