http://qntm.org/responsibility is an interesting go at it, and where I first learned of the concept. His other fiction is pretty fun, as well.
Afai(!)k, there might as well not be a top layer but infinite nesting -- or, even more charmingly, endless recursion, each simulation containing not another layer, but another iteration of itself. This at least would eliminate the embarrassment of being "less real" than the above layer.
--edit-- My own thoughts were along the lines of - humans decide we're probably in a simulation, and try to figure out how we could attempt communication with the simulators. To do so we decide to build our own simulation, then run it at double speed and watch how they try to communicate with us. Then recurse...
If you ever read a story like that, you'll know someone with the same idea but more motivation than me wrote it :)