A few months ago. Rémi Coulom's Crazy Stone engine had drawn quite a bit attention when it played at 4-5d on KGS. A Windows version was released around the time, but they also promised an iOS version for the iPhone. I finally discovered it a few days, although they apparently renamed it to "Champion Go" for the iPhone. For AppStore standards, the price tag of $7.99 is relatively steep, but it's still much cheaper than the PC version and not too far off from other Go playing apps.
Information and screenshots:
http://itunes.apple.com/app/id450726575?mt=8
I picked it up and have been playing on the iPad in x2 mode. It's a little fuzzy, but definitely usable. It supports 9x9, 13x13 and 19x19, three different input modes (zoom, cursor and touch), 10 different levels of playing strength on the 19x19 board (on 9x9 and 13x13 there are 5 difficulty settings), byoyomi for the player (I actually use and like this to force myself to play slower, but not too slowly), and it can both save and export (via email) SGFs.
And it has "medal challenges"! Altogether there are 150 medals that you can earn by playing against the bot on different board sizes, difficulty settings and komi/handicap setups. 100 of them are for 19x19, and 25 each for 9x9 and 13x13. Granted, this is a bit gimmicky, but it provides motivation and offers a challenge with "trackable" progress. It's tad video-game-y, like "achievements", but yes, I'm having fun earning medals and play the bot more than I would without these trophies.
An upgrade to a universal version with retina/HD graphics would be lovely, as would be a multiplayer feature with GameCenter support, but the latter is quite possibly beyond the app's scope! (Ironically, the prettiest board and stones I have seen on the iPad/iPhone are featured by an app that doesn't even play Go: Connect6, which is quite fun and very pretty, but it's not Go.)