Perhaps it was just coincidence, but the day before I wrote up my post documenting my build-your-own OTA DVR setup, Microsoft broke the free television guide service for many users. Apparently they switched providers from Zap2it to Rovi. I only noticed because my kids started complaining that none of their cartoons were being recorded anymore!
I found a workaround for this issue documented in a blog. However, my fix was a bit easier than what was described in that post – all I had to do was re-run the initial TV setup. This can be found by going to Tasks -> Settings -> TV -> Setup TV Signal
Re-running the setup takes a while (it performs a new channel scan), but after about 10 minutes everything was back to normal.
Hope this fixes things for everybody!