I'm going to swap my rear hydraulic brake for a mechanical brake today. I should be solid on the mechanical brake installation, but I have no experience with hydraulic brakes. Is removal as simple as unscrewing the hose from the caliper and lever and then pulling the caliper and lever? Will brake fluid go everywhere when I unscrew the hose?

Remove it as a unit, caliper, hose and lever. Why would you want to disassemble it?
I won't get into why you want to make the swap, but keeping it as an intact unit makes it more saleable, or easier to store, and no worries about fluid everywhere.

you shouldn't need to undo any part of the system, just cut off / undo the zip ties / retaining cips that hold the hose on.

Oh, interesting. It hadn't occurred to me it could be removed without disconnecting the hose. The brake is permanently damaged from what the bike shop told me, though, so I suppose it doesn't matter too much if I mess something up on removal.

I took the bike into the LBS because the brake was locking up in the heat. They tried to fix it, but ended up making it worse and said they'd have to replace it. But I need the bike working this weekend so mech brake it is.

Know, They were installed pre-bled, in the 1st place..

Well, the mech brake is installed and hydraulic removed. It came off pretty easy in one piece. I was surprised to see the frame braze-ons for cable routing were made specifically for hydraulic brakes. They were too large to stop ferules from passing through, so I had to just run one long piece of cable housing from the lever to the caliper.

The bike shop told me a new hydraulic brake would come in and I could come pick it up. Will it likely be the entire assembly: lever, cable, and caliper, or will it just be the caliper?

More and more bike are being spec'd with for full length cable housing now for both brake and gear, and if you have disc's on a frame specd for them you will only have the full length option.

For new brakes, unless you are buying them as individual parts, all hydraulic brakes should come pre-assembled and pre-bled (although some manufactures do the bleeding better than others) ready to fit / ride. Most although not 2012 Shimano will also come with a rotor.

That'll be nice, then. I could easily put the new hydraulic brake on myself once the LBS gets it in. But I probably won't bother...mech brake works good so far.

"...mech brake works good so far. " Let us know how it works in the rain.

I'd imagine the same as all the other mech brakes I've used in the rain--just fine.

you'd be right. especially on the rear.

Are cables somehow affected by rain? Please enlighten us.