Is there an iPhone App?
No. Unfortunately, there is not a native iPhone app for One tsp. You will not find it in the iTunes App Store.
You can create an icon on your home screen for One tsp. that will make it look very much like a native iPhone app.
Mobile access to your recipes is a core philosophy behind One tsp. It's why I created the app in the first place. So there is a very good mobile web interface for One tsp. that has nearly all of the features of the standard version of One tsp. you find on a desktop web browser.
To access One tsp. on your iPhone, Android, or other smart phone, open your phone's web browser and enter the address: http://onetsp.com.