Well, my CEL has been on for a couple of weeks now. A quick diagnostic the day that it came on told me that it was an O2 sensor heating circuit that was misbehaving, so it's A) not a critical problem, and B) only causes an issue until the exhaust gasses bring the O2 sensor up to operating temperature. I'll probably drive it like this for another month before I get an opportunity to take the truck into my mechanic's; no big issue, though.
It would be nice if the CEL had two levels of alert, one for minor issues such as an O2 sensor that, at worst, reduces engine efficiency, and major issues, such as a dead cylinder, regular misfire, or ECU problem.