While we haven't found a way for end users to easily add a word to the iPhone's dictionary, there's a great & NSFW post at T'N'T Luoma (be warned, there are saltier words than "hell" over there) describing a clever trick using your contacts list.
Of course, you could either click the little "x" to dismiss the suggestion or disable auto-correction completely (under OS 2.2), but what's the fun in that? We hope you find this trick handy. Just be careful when printing contact labels for your holiday cards.
[via Daring Fireball]