The Apple Store restocked the Apple MacBook Core 2 Duo 2.1GHz 13.3" Notebook in White, model no. MB402LL/A, for $849 with free shipping.

It's at the top of the page. (If it goes missing, that means that it's sold out.) That's the least expensive MacBook we've seen in quite some time.
It comes with a 1-year Apple warranty, the same as new Macs. Sales tax is added where applicable.

Update: Even cheaper, now at the top of the page is the $799 MacBook 2GHz, model no. MB061LL/A. Other refurbished MacBooks are available, also with free shipping, as we mentioned earlier this week:

• MacBook MB403LL/A 13.3" Core 2 Duo 2.4GHz Laptop in White for $949
• MacBook MB404LL/A 13.3" Core 2 Duo 2.4GHz Laptop in Black for $1,099
• see all refurbished Apple MacBook laptops (4 in total)