I have added a master suite onto the house including a master bath from the ground up. For the shower/tub, I put up .2 mil plastic to block drafts around the tub. Behind the plastic (in order) is the framing/fiberglass batt insulation, Tyvec housewrap, then James Hardie Hardipanel, caulked and painted. We have paperless drywall starting about 2 ft above the tub, and haven't put up the backerboard yet because there is moisture developing behind the plastic barrier. We have run the water in the tub a few times and some has splashed onto the plastic, but since its on the living space side, it doesn't answer why we have moisture on the other side.
The plumbing is brand new PEX using Rehau fittings, so I know they are not leaking. The shower valve body is a Moen and I capped off the tub diverter since we have a separate Roman tub faucet. The problem began when filling the tub from the Roman faucet, not the shower valve body, but the most moisture seems to be between teh studs where the shower valve body is. I also sealed everything with pipe dope wrapped in Teflon tape, since I don't like rework.
I hope someone has an idea how to help, because I don't want to rip down any drywall unless I have to.

Thanks so much.

I assume because your living in AZ that the weather has been real hot and because of recent rains you have been a little high in humidity.
The moisture is being created because of condensation. Your cooling the plastic because of the AC in the house to keep in comfortable and the outside of the plastic is warm with humidity. You are cooling the plastice down enough to condense what humidity you have in the wall. Kinda like a glass of ice water sitting for a while and moisture gathers on the outside.
If I remember correctly your vapor barrier should be on the exterior wall side of the wall not the interior side. You need to correct this as it will only get worse.
Look at it like this.
You cannot stop moisture (humidity) from traveling through the exterior wall. The house wrap allows for vapor transfer not water. Because of type of weather that is common for your area of the country the interior wall vapor barrier must be on the side with the most humidity and heat. The reason for this is to stop the humidity from traveling through the insulation to a point in which it will cool enough to condensate. In your case the moisture has not stopped and traveled through the insulation and stopped on the cool side of the wall. The bathroom.
Even if your walls were up this would still occur. Not in such quanities as I am sure your seeing but will happen to some extent non the less. If the entire home has been done this way (I hope not) this will occur throughout the home. The best way of telling if this is occuring is to get a moisture meter with the long probes. Remove outlet wall plates and test the insulation from within the walls for higher moisture content. You should get something in the area of 12% or less. Be sure you are not touching foil backed insulation when doing this or the number will be quite high.

Ok, so that makes sense, except that the condensation began when filling the tub, raising the humidity in the room. So that would mean that the plastic isn't sealed and humidity travelled outward then, right? So I rip down the plastic (no problem, just in the tub surround), then any recommendations for sealing the tub area against drafts? Its a spa tub, so multiple access points are key.

By filling the tub and raising the humidity within the room would mean that some of the moisture in the room could be part of the cause.

If you have drafts that means that the vapor barrier that is supposed to be on the other side of the insulation is non-existant. You should not have any need to access the tub on the exterior wall areas. I would suggest having a spray foam contractor come in to spray foam the walls on the exterior part of the wall. This product will not only provide you with a good insulation quality but also give you the vapor barrier you require.
As far as access I know of no tub that when placed in such a way that the exterior wall sections are modified to allow for access. In other words. You would have no need to go through the insulation to access any part of the tub. That would mean that you needed to go through an outside wall.

Now for the interior side wall.
Schluter makes a product call Kerdi This product is cement trowled onto the wall before the tile goes on. The the tile goes right over it. Its a moisture resistant product that prevents any leakage from the tile grout and tile itself from getting into the wall. You can order it from your local HD store or go to a tile supplier in your area and they can get it for you. It is NOT cheep, but it works real well.
Just remember however, you need to either install a vapor barrier on the outside of the framing, or use the foam insulation that acts both as a vapor barrier and insulation all in one. Do Not Use the foam that comes in the small can. Its not the same stuff.

Thanks, I was thinking foam under the tub on the exterior walls. Can I DIY the spray foam? If so, where to get it? I imagine its similar to the spray foam used instead of batts for vaulted ceilings and walls, or am I off base?

You would save more money if you had the insulation sprayed on by a professional. This stuff is not cheep when purchased in smaller batches. You would need a special spray device attachment that mixes the two chemicals together.
For the small amount you need I would have it done by someone else.
As far as getting it. Check with you local insulation supply company. You will not find this in the big box stores nor many lumber and supply yards.