I've noticed a couple of people on these forums looking for a way to switch off comments on old posts. (WordPress only lets you switch them off for new posts, at least up until v 2.0.3, it might become available in future versions.)
I wrote "Extended Comment Options" which lets you open or close comments and/or pings for all posts. You can also do stuff like switch them off for posts older than a year, or open them on the last five (or whatever) posts and close them on the rest, plus a whole bunch of other options.
Check it out: