As several commenters have reported, I’m not the only one who’s having problems. It looks as if Akismet is broken right now.
Meanwhile I’ve installed a new plugin, Bad Behavior, “a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots”. It’ll be interesting to see how well it works. If you find it blocking your comments, please let me know via email.