Qt has recently added support for mobile platforms, and has a C++ sensor API.
If you aren't already using Qt, you almost certainly don't want to include the giant collection of libraries that is Qt, but luckily, Qt is opensource under LGPL and GPL, so you could look to see how they do it, separate out their sensor code, wrap it in its own separate LGPL'd library, and then use that - dynamically linking to it, so the rest of your closed-source commercial product isn't hampered by the licensing.
Or you could just examine it, to see what API function calls they use, and then write your own wrapper (or better yet, submit a patch for SDL so others can benefit) by reading the API documentation.
The LGPL is incompatible with Apples TOS (for the app store) and dynamic linking isn't much of an option on iOS(it is possible but apps using dynamic linking will be rejected by Apple). so using LGPL code in mobile cross platform applications is a pretty bad idea unless you intend to only support jailbroken iOS devices.