I would not expect an HN username to do much good on a resume unless, as JM suggests, you are applying to a YC-backed company.
On the flipside, the kind of company that considers your HN username a compelling data point on your resume is probably a fun company to work for. :)
This is exactly why it doesn't hurt to include it. I live and work in London and if someone applied for one of my jobs and had their HN username on their CV, I would check it out immediately. The fact is, it shows an active interest in the field outside of your day-to-day work. I can't think of any circumstance where it would have a negative impact.
If you were to include your username in the "interests" section of your resume, I think it would do no harm if your comments are reasonable and could possibly help depending on the personality of the person who reads your resume.
Association says something about interests and character, I believe.
The question is really, do you want potential employers to read the comments you've made on this site?
GitHub or SO would be interesting, though.
Nothing I post here I'm really embarrassed about, but nothing I'm really proud of.