It doesn't have to be a reversible hash, just assign a guid for each ip address. There are probably subreddits where that information could be potentially harm

It doesn't have to be a reversible hash, just assign a guid for each ip address. There are probably subreddits where that information could be potentially harmful (eg upvotes in jailbait way back when) but that can be solved by not enabling it across the board, and it's not an issue here. Digg made their upvotes public from the start, and so do delicious, facebook, stumbleupon etc.