This caught me by surprise. I was reading pixelated image about creating a “Oh Sh!t Kit” and he talked about having replacement clock batteries. For some reason, I had never thought about it. When I pulled the battery out of my old D40, or my current camera, the D90, part of the display is still powered, and the the setting and clock don’t get reset. Obviously there must be another battery.
First thing I did was try to look at the manual. I found a PDF of the D90 manual that you can download here. I found on page 28: 
¬†”The camera clock is powered by an independant, rechargeable power source, which is charged as necessary… two days of charging will power the clock for about 3 months.”
 This sure sounds like it is not user replaceable. So I did some more searching, and found this flicker D90 group discussion about his clock battery dying, and no talk about how to replace it.
I then found this Nikon support entry on replaceing the battery in the D3/D3x. Those cameras use the CR1616 “watch” type battery. That is not a rechargeable type battery. Is the D3 the only Nikon camera that has a field replaceable clock battery?
So good news D90 (other Nikons ???)¬†owners, one less think to pack in the Oh Sh!t Kit, but bad news in that if it does die, you will likely say more than “Oh Sh!t”.
If you take it into any ritz camera they can usually take care of something like that for you, or just give you a straight answer on whether or not it can actually be replaced. to me though, its not that big of a deal if the battery dies for the clock. But that’s just my own personal preference.
While I am sure that ritz, or another store can send it to Nikon (you can replace it there?), not having a user replaceable battery is troubling to me. The timestamp from my camera is important for keeping an order to my images, especially shooting a wedding with multiple shooters. The timestamp is important to those using GPS tools to sync up the data. Other than the clock, it would be real annoying to lose all settings and custom menus you have in the camera if you changed the main battery, and the clock battery was dead. That I don’t know for sure, but not sure what would power the memory if this “clock” battery was dead.
i think im joining the rank of Oh Shit my internal battery has died. writing to Nikon now to wait for their response (im on the road so cant detour to a service rep)