Typically, at least in the past, when someone did a search and the top two results were from the same site, Google would indent the second result under the first. It works sometimes now, for example a search on rustybrick indents the 2nd result as such.
But a search on webmasterworld does not indent the result.
An other thing which doesn't appear to be fully consistent. What I did notice is that the results with the Web Categories underneath them, tend to not have the indented result after them. So maybe that is the difference? See google for an example like the rustybrick example above, see no, web categories...
Forum discussion at WebmasterWorld.