The assumption here is that, during Microsoft's reactionary quest to address security flaws, someone broke EVENTLOG.DLL. I suspect that my event logs began to disappear after an automatic update from Windows Update. Some guy named Matt posted this on bmonday(dot)com:

"I had this problem and it only reared its ugly head when I changed the log files to 'Overwrite as needed' because I got fed up of deleting them. So this supports your theory."

So what I did was set all of my logs to Properties > General > Log size > Do not overwrite events (clear log manually). Microsoft recognizes this problem in MSKB article 829246 ("Event logs are corrupted") here:

http://support.microsoft.com/?kbid=829246