The ProjectionPartDriver excludes all content items whose type has a ProjectionPart attached to prevent an infinite loop.
Why this needs to change / Use Case Scenario
I have a single page website where all pages are projected using a Query and a ProjectionPage.
The Query includes all Content Items whose Content Type has the SlidePart attached.
One such Content Type is called a Speakers Slide, which has a ProjectionPart attached that projects a series of Speaker content items.
How it could be solved
One way that is suggested in this thread: http://orchard.codeplex.com/discussions/391631
is to use a counter, limiting the number of nested projections.
I didn't investigate what exact conditions would cause an infinite loop, but if we know what those are we can mitigate them in a more flexible way so that we can at least support nested projections.