The move is said to have been made by Nokia because of the non-upgradeability of the Lumia 900 to the upcoming Windows Phone 8 OS, and only to Windows Phone 7.8 instead from the current WP 7.5 OS.
Unfortunately, most articles on the top have focused on the headline, upfront price reduction of the Nokia Lumia 900, although their articles usually do refer to the fact that there's still 24 months worth of payments to be made before the phone is yours outright.
In Australia, some carriers offer smartphones at a $0 upfront cost, but that does not make them free, of course.
You’re still paying the cost of the smartphone in question over what is usually a 24 month contract, with part of the monthly price the “minimum” you pay for your phone plan, and another part of that monthly price goes towards paying off the phone.
The move has obviously been made to ensure consumers still take a look at the Lumia 900 when comparing it against an Android smartphone, an iPhone, a BlackBerry or something else, and has been made in concession of the fact that post the update to Windows Phone 7.8, there won't be any upgrades to future OS versions.
There migth be patches and security updates, but the focus will have shifted to Windows Phone 8, from the OS to apps, thus leaving Lumia and other WinPhone 7.x users on a path toward eventual oblivion.
So, if the so-called 50% price cut of the Nokia Lumia 900 on AT&T interests you, by all means take a look, but be aware that your app choices will likely shrink on WP 7.x faster than for Androids and iPhones, while Windows Phone 8 gets all the love instead.
50% off? No, it's just a $50 discount, and that's a very different kettle of fish.