If you want to save some money (and assuming by "flip-flop" you mean fixed/free) you can get any old Shimano disk hub, use a single speed cassete cog and spacers on the free hub side and a bolt-on fixed cog for the rotor mount. The cogs are available from a few places online, but if you know a machinist who works for beer, you can get him or her to mill out the splines just a bit and drill holes matching those in a disk rotor.
I've been using this setup this winter and I have no complaints. Chainline is fine, and to me, an XT hub is a better product than the Surly hub and much cheaper, even counting the cost of shipping or fabricating the special cog.