UnitOfWork in Action Filter seems to be caching

I have an MVC 3 site that uses IoC (Unity), and my model is generated w/ EF4 and POCOs. I am using an action filter to commit my UnitOfWork:

public class UseUnitOfWorkAttribute : ActionFilterAttribute, IActionFilter
{
    private readonly IUnitOfWork _unitOfWork;

    public UseUnitOfWorkAttribute()
    {
        _unitOfWork = IoCFactory.Instance.CurrentContainer.Resolve<IUnitOfWork>();
    }

    void IActionFilter.OnActionExecuted(ActionExecutedContext filterContext)
    {
        _unitOfWork.Commit();
    }

    void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)
    {
    }
}

However, even though the Commit() seems to be getting fired, it somehow seems to be caching what it thinks is "dirty".

For example, in my controller, the following gets executed from a service class:

var user = _userRepository.Single(u => u.Id == 2);
user.DateAdded = DateTime.Now;

Whenever I do a fresh build of the solution and hit this controller action, the change is actually committed. However, successive hits to the controller doesn't do anything.

On the other hand, if I put a UnitOfWork in my controller and commit it following the service method call, it works as expected (every time I request the controller action):

public AccountController()
{
    _unitOfWork = IoCFactory.Instance.CurrentContainer.Resolve<IUnitOfWork>();
}

public ActionResult Test()
{
    var user = _userRepository.Single(u => u.Id == 2);
    user.DateAdded = DateTime.Now;
    _unitOfWork.Commit();
}

So it definitely seems like some sort of caching is going on, but I can't figure it out what is getting cached -- the UnitOfWork, the ActionFilter, or the repository.

Any ideas what could be going on? And if not, any ideas what else I could do to troubleshoot?

Thanks in advance.