As if the current crop of virus warnings and financial data-stealing Trojans weren't enough, mandatory security updates just came out for both Movable Type and Drupal. The Movable Type upgrade went without incident but Drupal kept reporting that the installation was not up to date in the status report page.

It turned out I had a slightly altered version of the core Aggregator module in the sites/all/ modules folder. I updated that, and did a difference search with BBEdit to find the line I'd changed and restored the change. That did the trick - Drupal realized it had been updated.

Thanks to Jenny Simonis for the tip that helped solve this one.