...and the reason is simple: The supply of old pennies appears to have dried up in my usual haunts. I got a 1967 penny at Fry's on the 18th, and that was it. Nothing I've gotten since then has been older than 2003.
So maybe it's a local rather than a global phenomenon: Somebody cashed out a big penny jar, and it's taken until now to work through all those 50-60-year-old pennies. I'm going to keep watching, of course, but unless the trend appears again I'll assume it was a one-time thing.