"If these rumors are true, buying a Lumia 900-or any other Windows Phone handset-is a risky proposition. If you buy subsidized, on-contract handsets, you'll typically be eligible for a new subsidized upgrade every two years. A reasonable expectation, then, would be two years of software support in the form of bugfixes and updates. But if current handsets can't run Windows Phone 8, they'll essentially be cut off from future support in about six months. Developers might still target the older handsets to some extent, but many more will be targeting Windows Phone 8's extended capabilities and richer APIs."
If you are contemplating buying a Windows Phone it will be of interest to you to understand the update strategy from both Microsoft and the carriers proving the phones. As is outlined in this article, it is clearly an uneven playing field and playing strategy from the participants. Ars Technica goes so far as to warn you that buying a Windows Phone like the Nokia Lumia 900 could lead you down a path where you cannot get your phone software updated, depending on which partners are involved. Note that the Android world also has some of these types of issues. It is an in-depth article, but well worth the read, and only takes one click to get there.