Two more interesting options for a prefab cabin:
Panel Concepts has a number of 'panelized' cabin kits from 12' x 12' bare-bones shells to 20' x 40' insulated and partitioned units. Pro: Whatever you think of the faux log cabin siding, the reported assembly time is one of the lowest I've seen. Con: The nearest distributor with an assembled sample is 5 hours away in upstate New York.
Zook Cabins builds and delivers completed cabins from 12' x 18' to 22' x 48' to your [future] doorstep, taking the 'you' out of DIY. Throw a little more money at them and they'll install a bathroom and custom kitchen for you. Pro: Very competitively priced, can combine trip to the factory with a weekend at Sesame Place for The Boy. Con: I'm always suspicious when a manufacturer claims 'Amish Craftsmanship' on a product that no Amish would ever buy.
Another interesting option came at the suggestion of the LSW: Craig's List. Sure enough, there were a number of barns and antique post-and-beam frames for sale, and even a listing for historic Appalachian log cabins ready to be reassembled on your site (banjo not included).