"Skyrim uses a lot of dynamic streaming systems, including textures," says a Bethesda community manager posting on the official forums. "We've seen a few reports of certain textures temporarily scaling down on the Xbox 360, and not scaling back up. We have verified that this issue does not occur when playing off the disk and when the game is fully cached (not installed)." For now, it seems the streaming glitch can be avoided if you play directly from the disc, and imagine the loud whirring emanates from Dovahkiin's vocal chords warming up.
According to Bethesda, a solution is being prepared for installed Skyrim in the next title update. The patch notes for the most recent update are available here.
* Sorry, we just wanted to share that we've been thinking about turn-based tusk battles between customizable mammoths all afternoon.