Hi Marty,
Absolutely, breaking the constants out into separate files can make sense when their volume reaches a certain point.
It could also be that breaking the application itself into separate apps could be indicated at this point as well.
If you decide to investigate that route, you might sign up for the PureMVC Courseware beta test. You'll work through a set of apps using the same Model, each with their own Facade and use cases against the common Model.
And also you might have a look at the work Nate Rock has done with Modules:
http://forums.puremvc.org/index.php?topic=49.0-=Cliff>