Alternatively, would you take steps to limit new membership? HN these days is often mentioned in the same breath as Twitter, Reddit, and Digg by many bloggers. I don't see that trend stopping any time soon, so if you didn't plan on implementing subreddits, then something like this might be necessary.
The Eternal September phenomenon has always been fascinating to me, not least because it is one of the Internet's oldest problems, yet it remains unsolved. I'd be interested in reading your thoughts on how a community can protect itself from dilution in this manner. Beyond the ideas expressed in the following link, did you have any other thoughts? Has anything changed your mind in the past year?