LOL - actually I asked my wife about it last night... Here is what she said... Sorry for the delay getting back on this... I don't hang out in spas much!
The Spa at Camelback Inn (Scottsdale) is supposed to have a pretty nice spa - supposedly all kinds of celebs go there.. the hotel is a little bit older but it has a PIMP pool with giant rocks and waterfalls.... Very arizona feel to it...
http://www.camelbackinn.com/index.html
The Boulders Resort (Carefree) is also pretty sweet according to the wife... Also it is like a crazy little cowboyish town up there, so Carefree is interesting in its own right...
http://boulders.wyndham-hotels.com/
Those are the 2 she first thought of, but here is a listing of more if you want to call around...
http://www.arizonaguide.com/spa_guide.asp
of those listed, The Sanctuary on Camelback Mountain, The Centre for Well-Being at The Phoenician, and The Spa at Gainey Village are also most likely some of the nicest places as "The Sanctuary" "The Phoenician" and "Gainey Village" are some of the nicest places around....
weblinks and phone numbers on the links...
If you go soon you might get a better deal as summer months are "off-season" here, but between you and me it hasn't been as hot this year as usual.... I would make sure to ask about seasonal adjustments to rates and if you would get a better deal now vs later...