Reporting on nested projects

I'm trying to create a report that outputs all the Timesheet entries for a particular project. The complication is that the project contains sub-projects, and those sub-projects could also contain sub-projects (and so on...)

Is it possible to design a report where the user picks the top-level project, and all the timesheet entries for the nested sub-projects are reported?

I can design one which works for certain levels of nesting, but not one for any number of levels.