Once I looked into the code further, it's apparent that there's no 
need to change Group.php or DataTree.php - we just have to call 
getGroupName() in Whups. Changes committed.

Both yes. We have other cross-package dependencies, but those doesn't 
matter as long as the dependencies stay inside a (release) module or 
in a not yet released module.

Just to check before committing this - are we okay with this meaning 
that Group.php would require functionality that wasn't in Horde 3.0? 
And are we okay with Whups requiring Horde 3.1?

Attached is a cleaner patch that will traverse all the way to the root parent.

makes sense to me.

This could maybe even be the default behaviour. Other opinions?

That patch doesn't really help, because it only adds one level of 
parent groups, but you can have an arbitrary level depth.

Also, you could save a lot of redundant code of you add an optional 
parameter to getGroupName() instead that makes the method return all 
parents with the group names.

Currently, the group names listed throughout whups don't display the 
parent group names as part of the group name.  This can be confusing 
especially if two different parents have a child with the same name.   
Attached is a patch that fixes this.