If you're building mobile related retail applications or extensions to your ERP/Ecommerce platforms on iOS for the iPhone or iPad, then you're in luck. We just completed the iOS Software Development Kit that allows you to easily connect to Avalara's AvaTax API. The SDK comes as a static library that you can link to your iOS projects. For REST de/serialization, we've used a 3rd party library called JSONModel which simplifies data transformation from object model to JSON (and back). We've included that into the project as well as the full source code under the Apache 2.0 license.
You can download the SDK and others from our Sample Code Library. Please feel free to try it out and give us some feedback.