HJ, I did just that. And, yes, I think it's threaded because you can see a hex copper (or brass) fitting in the photo (above) very near the outlet of the spout (near the diverter assembly). I had one of my sons turn the spout while I watched from the hallway (from the same location as the photo above) and, no, the copper didn't rotate. What DID rotate is the spout (obviously) but also the white plastic fitting (fluted, I think--look closely at the photo) that you can just barely make out (part of the diverter assembly, I think), at the farthest point from the camera, right near the outlet of the spout. If you look closely at the photo you can see what looks like 4-5 tiny radial cracks (maybe 1/8" long) in the fluted (white plastic) fitting, spaced anywhere from 1/4" to 3/8" from one another. When the spout is turned, the fluted (white plastic) fitting slips over a mating fluted surface (presumably brass). In other words, I probably stripped the plastic flutes off when I turned the spout with the strap wrench (or they slipped because of the radial cracking which, again, I may have caused when I turned the spout with the strap wrench).
You may be correct about the Moen model number 3811. It sounds about right. (I had to get parts for the mixing valve about a year ago which is the only reason it sounds familiar.) I can confirm it is a Moen, it has a shower diverter and it's probably 33 years old (original to the house).
I removed the bottom two courses (4" + 4" = 8" total height) of ceramic tile and drywall around the entire perimeter of the tub due to moisture wicking up from the failed silicone butt joint where the (cast iron) tub flange and the tile met. In other words, I have complete access to the spout and service piping from the hallway access panel side (inside the wall) and from the bathroom side (standing in the tub). The mixing valve I only have complete access to from the hallway access panel side since the drywall and tile are still sound and intact at the height of the mixing valve and weren't removed.
My plan is to replace the 8" of missing drywall and tile that I removed (x 1/2" + 1/4") with 3/4" of cement board, sand/abrade the remaining ceramic tile for better adhesion and then apply the mfg.'s recommended mastic (water-based Liquid Nails FRP 'professional' adhesive) with a 1/4" notched trowel before finally installing an $80 Home Cheapo 5-piece plastic tub surround. I have a son returning from Afghanistan in Aug. so I want to make the shower presentable, at least. (I start my new job in 2 weeks but I've been unemployed for 1 1/2 years so money is tight. When I can afford to pay for a more professional looking surround--and possibly a new shower/tub fixture assembly--I most definitely will!)
Anyhow, still wondering if I should attempt to pull the spout straight out (and off) or should I sweat the solder joint at the first copper elbow. I can't think of anything else to remove it other than cutting the 1/2" copper pipe right after the elbow, just before the pipe starts the 4"-5" run inside the spout.