Official XBMC Remote for iOS allows you to control your XBMC media centre via iPhone / iPad. This app lets you browse the library, view information about the item that is currently playing and direct remote control. If you have multiple PCs running XBMC, you can create connection for each one of them.
The app is free on App Store. There are in-app instructions on how to configure your XBMC so that remote control can connect to it.
More information about the app is available on the official page.