Near the end of April, I installed the Anti-spam plugin, which added an automated JavaScript-based verification system. The effects have been drastic:

The effects of Anti-spam

This graph shows the amount of spam that has been reaching Akismet: absolutely none. At the same time, legitimate comments have been making it through, so the system seems to be working as expected. I highly recommend this approach to combating comment spam, at least until spammers find a way around it.