According to the Constraints Guide, ISE supports the creation of AREA_GROUPs from TIMEGRPS; however, when I open PlanAhead on a design that has AREA_GROUPs defined that way, I do not see them in my Physical Hierarchy. Are these constraints valid? Will they be passed to my output ucf?
These constraints are valid and supported. It is a bug in PlanAhead that they are not visible there. These constraints will be passed to your output ucf.
This is scheduled to be fixed in the 12.1 PlanAhead release.