Interface implementation and covariance

I was a bit disappointed to discover that the following code doesn't compile in C# 4. In particular, it seems that as the 'Thing' property is only a getter on the interface, any concrete implementation should be able to return a derived type, since it can be guaranteed that anyone getting the parent type from the interface will also be able to operate on the child type.

class Thing { }

class SpecificThing : Thing { }

interface IThingContainer
{
    Thing Thing { get; }
}

class SpecificThingContainer : IThingContainer
{
    public SpecificThing Thing { get; set; }
}