Yea this is one genre iOS gaming is sorely lacking. I think FF Tactics is coming this spring for iPhone, not sure about iPad. Until then, closest thing I found is Highborn HD, but it plays more like Advanced War.
There are several story-based tactical RPGs in the App Store, but none of them are exactly like Jean D'Arc. The closes is probably Shining Force, which is a decent port on the Sega classic. There's also War of Eustrath, which plays more like Super Robot Wars -- mecha combat with a bit of fantasy thrown in. Both those games are top down, rather than isometric. Tactical Soldier: Undead Rising is isometric, but plays more like a combination of X-Com and Resident Evil -- it's story driven, but the RPG elements more muted and the narrative doesn't have the epic sweep of games like Jean D'Arc. Then there's Shrowded Aspect, which is probably the closest to Jd'A thematically, but doesn't have the sane kind of strong character progression. There are other turn-based tactics games that are not story driven: Hunters and Return of Fortune are two that cone to mind. Hopefully FFT will arrive earlier rather than later in the spring. There are also several other turn-based tactics
