It's an interesting idea, but honestly, I find any boilerplate claim to ownership of content posted on a site or imposition of a license, well, frustrating.
The magazine is fantastic and I completely support it, but I do think this should be an opt-in. Perhaps it would be easier for commenters who wouldn't mind their work being published elsewhere to just drop a note like "All comments licensed CC/NC/SA" in their profile?
IANAL, but by posting here you necessarily give HN an implicit license to distribute your comment, roughly equivalent to the legal boilerplate you'd find on YouTube or similar ("worldwide, non-exclusive, royalty-free license to redistribute, perform," etc). I don't think making it explicit with Creative Commons would be an "imposition". Creative Commons wouldn't give HN or anyone else ownership of your comment.
Opt-in wouldn't solve the problem for Hacker Monthly, as few people would bother. Opt-out would be fine. BTW, CC/NC precludes inclusion in Hacker Monthly as it is sold and contains advertisements.
I agree an opt out option would be best. Basically under each comment box a text that says:
>>
By posting here you are making your text available under the Creative Commons Attribution license. If you wish to not provide your text under the Attribution license please check here. To not see this message again check here. (you can change these settings in your profile at any time).
The magazine is fantastic and I completely support it, but I do think this should be an opt-in. Perhaps it would be easier for commenters who wouldn't mind their work being published elsewhere to just drop a note like "All comments licensed CC/NC/SA" in their profile?
(I've dropped this in mine).