This is the second time I've upgraded Community Server and forgotten to re-enable ReverseDOS in the web.config file.

I can tell straight away that it's not working because of the barrage of spam comments that appear overnight.

CS 2.1's new spam filters are pretty good - they seem to catch a large percentage of the spam - but ReverseDOS is damn near infallible. Well worth checking out if you have a comment-spam problem on your blog (whether it's CS or otherwise).