Janine, although the datamodel supports the use of subprojects, the UI does not. It is not going to be terribly difficult to add back in support for subprojects, but it will take some hacking. There are likely a bunch of small bugs that will have to be fixed.
I'm not using subprojects at my installation, but the underlying design took them into account, and a lot of the procs were written to support subprojects. I just haven't tested to make sure they work, which is a sure sign that they won't (!) without some bug-fixes.
Hope that helps.
