A brief recap of ASP.Net cache:

//in MVC use HttpRuntime.Cache or HttpContext.Cache
//in webforms Application is the original cache without expiration rules
var category = HttpRuntime.Cache["Category"] as CategoryModel;
if (category == null)
{
     category = _dataAccess.Find(1);
     HttpRuntime.Cache["Category"] = category;
     // ...or...
     //monitor some files and/or other cache items
     var cd = new CacheDependency(new[] { @"C:\triggerFolder\" }, new[] { "OtherCacheItem" });
     HttpRuntime.Cache.Insert("Category1", category, 
               cd, //dependencies or null
               DateTime.Now.AddMinutes(5), //absolute expiration (or Cache.NoAbsoluteExporation)
             Cache.NoSlidingExpiration //sliding expiration (timespan)
     );
}

Mocking this in tests (especially for MVC) is a bit ugly (.Net 3.5sp1 has System.Web.Abstractions including HttpContextBase, but caching isn't included)

Now in .Net 4 we can reference System.Runtime.Caching.dll. And the really nice thing is this will run outside Asp.Net.

//get the static "default" cache. You can have multiple named caches.
ObjectCache cache = MemoryCache.Default;
//you can't store null in the cache
var category = cache["Category"] as CategoryModel;
if (category == null)
{
    category = _dataAccess.Find(1);
    cache["Category"] = category;
    // ...or...
    var policy = new CacheItemPolicy();
    policy.AbsoluteExpiration = DateTime.Now.AddMinutes(5);
    //monitor some files and/or other cache items
    policy.ChangeMonitors.Add(
        new HostFileChangeMonitor(new List<string> { @"C:\triggerFolder\"})
        );
    //synchronize with another cache item
    policy.ChangeMonitors.Add(
        cache.CreateCacheEntryChangeMonitor(new [] { "OtherCacheItem"})
        );
    cache.Add("Category1", category, policy);
}

For simple caching (no change monitors) you don't even need mocking in your tests - in fact, you can test your caching with a real cache. You can move caching down into your library classes that may be called from web pages, tests, WPF apps and consoles.