Within the past couple days, the Lumia 635 has made a trip to FCC for certification, making the release imminent!
The device is basically a variant of the Lumia 630, the first Windows Phone 8.1 device to see store shelves. Internally they are pretty much the same, but this version is 4G-LTE enabled and only comes in a single-SIM variant, whereas the 630 only uses 3G and comes in both single and dual-SIM.
No release date has been given, but we should expect to see the 635 by this summer, and possibly as soon as next month. In the meantime, check out my hands-on video from the Nokia #moreLumia event back in April.