It would be nice if FilePathResult supported ETags and byte ranges. iOS requires byte range support for videos and caching is an important performance feature.
I've created a FilePathExResult class that handles it if you want to take a look at it. It handles ETags, LastModifiedDate, and MaxAge caching as well as byte ranges.
The code is at http://bizark.codeplex.com/
and the class is located at /Current/BizArkWeb/MVC/ActionResults/FilePathExResult.cs (plus other supporting classes).