The information I have relates to the US market only. These plans will be officially revealed at the Consumer Electronics Show (CES) in Las Vegas next month.
Three LTE-based Windows Phone handsets--the Nokia ACE, HTC Radiant, and Samsung Mendel--will ship on AT&T Wireless before the middle of 2012. The ACE is due March 18, 2012.
What's unclear is whether these LTE based devices will be accompanied by a software update of some kind. You may recall that the rumored "Tango" release, supposedly due in early 2012, is about low-end phones. So it's unclear how that jives with LTE, which is obviously a high-end feature. This explains my confusion over the supposed Windows Phone roadmap leak yesterday.
Also, the Nokia Lumia 710 will ship on Verizon Wireless in April 2012. This is not an LTE device, and it's not clear when/if we'll see an LTE Windows Phone on Verizon. The Lumia 710 is codenamed "Sword" and will be available in April 2012. (The T-Mobile version of the Lumia 710 will be available for purchase on January 11, 2012, giving the company three months of exclusivity before the device hits Verizon.)