Thanks, Shawn. I hope this is resolved soon, as 40% of what I watch on the Roku is via USB. If the problem persists, then the pleasure this product delivers to me will be about 60% what it was before yesterday.
Here's some additional info:
Not merely 4:3 titles are affected. 16:9 titles are also affected in the same way.
Not every video I tested was affected. Of 12 .mkv files I tried, seven were distorted (presented in what appears to be an extreme 4:1 aspect ratio), and five were unaffected. The video codec was the same for all 12 files. However, I found two common links among the affected files. These factors seem to reliably predict whether a file will be affected:
 The video is from a PAL DVD source, with a frame rate of 25fps.
 The video is full frame/4:3 from a DVD source (either NTSC or PAL).
4:3 displayed fine (a) in two cases where the original source was 1080p Blu-Ray, and (b) one case in which the original source was a laserdisc, subsequently converted to .AVI and finally to MKV. In the other two instances, the original sources were (a) 1080p Blu-Ray with an AR of 16:9, and (b) NTSC DVD with an AR of 16:9.
I performed additional analysis with VideoLAN as well as with an MKV analyzer and compared values, and this yielded no insight.
All 12 videos I tested were videos I'd watched via the Roku's USB input within the past month, at which point(s) they looked fine and none were distorted. The first thing I did after being confronted by this problem was check how recently the Roku Firmware had been upgraded, and I was not surprised that it had been updated merely two hours earlier.