The problem with accessing dependent picklists is the way they are stored in the schema.  Each dependent picklist entry has a property called "validFor" which is a bitset where each bit indicates whether the value is valid for that bit's position in the controller field's options array.  So if bit 0 is set, then the value is valid if the first choice is selected in the controller field, if bit 1 is set, then the value is valid if the second choice is selected in the controller field, etc...

Here is an example from a class that I use to get dependent picklist values into a readable XML representation.  It uses a helper class called Bitset (posted below) that does the work of breaking the validFor property down into an array or bits.  Arguments: picklistEntry is the entry to test, parentField is the entry's field, and strObjectType is the name of the object that contains the field.  Utility.GetField is a helper function that returns a field definition using describeSObject.

      private string GetValidForNodes(SforceReference.PicklistEntry picklistEntry, SforceReference.Field parentField, String strObjectType)
      {
         String strResult = "";

         // Get the controller field by name.
         if (!parentField.dependentPicklist) return "";
         if (parentField.controllerName == "") return "";
         SforceReference.Field controller = Utility.GetField(strObjectType, parentField.controllerName);

         // For each PicklistEntry get all controlling values for which it is valid.
         Bitset validFor = new Bitset(picklistEntry.validFor);
         if (validFor.size() == 0) return "";
         if (controller.type.ToString() == "picklist")
         {
            // The controller is a picklist.  Get all controlling values for which this entry is valid.
            for (int i = 0; i < validFor.size(); i++)
            {
               if (validFor.testBit(i))
               {
                  // Since the bit is set, this entry is valid for the controlling entry at index i.
                  strResult += "\t\t\t\t\t<dependentValue>\r\n";
                  strResult += "\t\t\t\t\t\t<value>" + Utility.XmlEncode(controller.picklistValues[i].value) + "</value>\r\n";
                  strResult += "\t\t\t\t\t\t<label>" + Utility.XmlEncode(controller.picklistValues[i].label) + "</label>\r\n";
                  strResult += "\t\t\t\t\t</dependentValue>\r\n";
               }
            }
         }
         else if (controller.type.ToString() == "boolean")
         {
            // The controller is a checkbox.

            // If bit 0 is set this entry is valid if the controller is not checked.
            if (validFor.testBit(0)) strResult += "\t\t\t\t\t<dependentValue>unchecked</dependentValue>\r\n";

            // If bit 1 is set this entry is valid if the controller is checked.
            if (validFor.testBit(1)) strResult += "\t\t\t\t\t<dependentValue>checked</dependentValue>\r\n";
         }

         return strResult;
      }


   class Bitset
   {
      // helper class to decode a "validFor" bitset class Bitset
      byte[] data;
      public Bitset(byte[] data)
      {
         this.data = data == null ? new byte[0] : data;
      }
      public bool testBit(int n)
      {
         return (data[n >> 3] & (0x80 >> n % 8)) != 0;
      }
      public int size()
      {
         return data.Length * 8;
      }
   }