System.Runtime.Caching.dll does not exist in the mobile (SilverSight Superset) profile used in MonoTouch (and Mono for Android).
supported in Mono 2.11 (unstable), so it may become available in the future. IOW MonoTouch will be overfulfilled on Mono 2.12 (i.e. 2.11 upgraded to stable). MonoTouch is now following the stable branch of Mono 2.10.
Of course, since Mono is open source, you can compile / try it in your own application.