"choices" for properties and action params could be changed to accept "Collections" and not only "Lists"?

[GESCONSULTOR - Óscar Bou]

Currently there is not possible to return a Set to the choices of a property or 
param action.

As with JDO more and more collections are implemented as SortedSets, we must 
"copy" all Set contents to a List.

If instead of requiring Lists to be returned,  "Collections" woul be allowed, 
that would be enough.
 
Is there any other constraint to consider?

Thanks,

Oscar