AT&T spent more on the Lumia 900 launch than they did on the iPhone launch in 2007, but the Lumia 900 sold poorly.
They're still the Windows Phone leader, so they're going to push hard, but it remains to be seen if they'll have any more success than the 900 delivered. Every day WPs are not sold is a day when an interested customer in his upgrade cycle gets a shiny new Android or iOS device instead.
T-Mobile has always been a Windows-oriented carrier (even back to the old days), but the Galaxy S III is their hero phone (along with the Note II). They'll no doubt offer the 8X and perhaps another handset or two in order to have a Windows Phone option set for customers, but their pitch is primarily around the network and not the device (hence the big push for unlocked iPhones).
Verizon is the wild card. Will they push WP? Depends. Android was big for them against the iPhone (since Apple wouldn't sell them the device at the time), but since then, they've grabbed the iPhone. Windows Phone has lower subsidy costs, but so does Android (and right now, people are buying all the Galaxy S IIIs that Samsung can make). They also have an agreement with Motorola to sell a lot of Droid handsets in exchange for exclusivity, which likely impacts their ability to shift stuff from Nokia and HTC as "favored" handsets.
At the same time, it's in VZW's best interest to have a third platform emerge. But they're hedging their bets -- they're also going to be the "big launch carrier" for BB 10.
We live in interesting times!