I don't think touch screens and Touhou get along very well. I do know there are Cave STGs on the App Store, but IIRC the gameplay is optimized for the touch interface and the games on the mobile devices aren't exactly the same as their arcade/console counterparts.
If Touhou was on iOS, I could see the main screen being centred and the controls on the sides to minimize screen blocking, but the game itself would probably be made easier owing to lesser precision of touch screens. I wouldn't be surprised if the iPad version also had BlueTooth keyboard support..
More importantly, IIRC ZUN doesn't allow Touhou derivative works to be made available on direct download services, including the App Store, Xbox Live's Indie Marketplace, and the Android Market, as noted on
his blog (translation/opinion on the issue
here (ugh Wikia) below in Master Bigode's link).
Therefore, I would recommend just making the PC version and deal with the touch-screen elements later (mostly for those pesky keyboard-less tablet PC users
). Touhou iOS will probably never happen.
(Oh, and I don't have an iOS device to test with, though I am also writing a game for said platform as well as PCs)