There’s an update on movabletype.org: News with a new Movabletype version now available — version 2.66. This update fixes the comment flooding issue that has been cropping up lately on numerous sites. Kudos to the Movabletype team for their quick action on this issue!
“In recent weeks, open comment systems, and Movable Type weblogs in particular, have become a target for comment spam and comment flooding (automated scripts that send many comments at a time to a weblog entry).”
“As a stopgap before we release comment registration in Movable Type 3.0, we’ve released version 2.66 of Movable Type, which includes some protection against comment flooding. We’ve included a throttling measure so that comments from the same IP address can only be posted every N seconds, where N is configurable.”
Update 2: Just saw Movabletype released a new version, 2.661, to fix the XHTML bug that breaks validation in the comment redirect script, along with using comment IDs instead of URLs to fix an open redirect problem.