In Nintendo's official forums, the word is that it's taboo to post one's Friend Code publicly, or indeed even to help board members find other places to share their codes.
Doubtless this is part of Nintendo's family-friendly privacy strategy, but with a game that permits random matching, surely the potential damage done is no worse than stumbling into someone with lovely hand-drawn genitalia for their kart emblem? We have no such problem with hands being tied in our forums; we've even given members the option to list their Friend Codes in their posts by adding the code to their profiles.
Presumably, it's still okay to send each other private messages containing a Friend Code on those boards, but you never know...
UPDATE: Thanks to reader Joe, who brought to my attention a detail I apparently skimmed over -- Nintendo's forum administrators also say that forum members are not allowed to share Friend Codes via private messages either! Which leaves me wondering: how does Nintendo intend to actually build a Wi-Fi Connection community?