Starting May 20, AT&T will sell a Samsung phone that runs Windows Mobile 7.5 and can use LTE service, where available. The Samsung Focus 2 costs $49, plus a two year contract. The Focus 2 has a 4 inch Super AMOLED screen, but resolution is limited to 800x480. The processor is a single core 1.4 GHz model.
This is the cheapest LTE phone available so far in USA. AT&T doesn't have a widespread LTE network, but if you live in the one of the company's LTE markets, this phone is a deal. Go to http://www.att.com/network/ and click on "Coverage" to see if AT&T is in your area. When not on LTE, the Focus 2 falls back to HSPA+, like the iPhone 4S uses.
Update: Better wait until we know for sure that this phone can be updated to Windows Mobile 8. The new version should be available by year's end. No sense in buying a phone that has no upgrade path.
Update 2: Some reviewers say the Nokia Lumia 900 LTE is available on contract for as little as $50, making it a better deal than the Focus 2. Here's one example
Wait, the Lumia 900 may not be upgradeable to Windows 8 this Fall, either. What to do, what to do?