Maybe you're already aware of this, but I've been noticing a lot of spam creeping into the comments for older articles. For instance:

/modules.php?op=modload&name=News&file=article&sid=906

I don't know if there's much you can do about this, but I figured you might at least like to know about the problem. It might be worth considering locking the comments on articles that are past the age where legitimate readers are still posting comments.