Hi,
A .NET defined LISP function (with the LispFunction attribute) can return any type of value available in LISP : string, int, double,Point3d, SelectionSet, ObjectId (ename), TypedValue (T or nil), ResultBuffer (list)...
But it seems there's a bug with AutoCAD 2008 which only allows .NET defined LISP functions to return a ResulBuffer (a list or nil).