For a week or so now, since the off-schedule high-priority Microsoft updates 2-3 weeks ago perhaps, I have had a six-month young Windows 2008R2 Standard (x64) machine (Hyper-V image) that refused to download updates. Despite doing some minor cleanup, service tweaking, rebooting a few times, it was just plain stuck.
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I didn't see any apparent system errors or Windows Update specific entries in the Event Viewer - Windows Logs, System or Application. There were, however, a number of TrustedInstaller errors:

Though I had not had the "opportunity" to experience an issue with Role Management, I was simply concerned with getting Windows Updates back to up-to-date, after days of frustration I came across this MSDN forum post that led me to a working solution.
I worked through all steps of the post marked as answer, finding that an in-place "upgrade" was apparently the only solution that worked in my scenario. This is rather disappointing -- up until now, Windows 2008R2 had never failed me.