Hi,
As far as I know DataTable are not serializable in Net 1.x. The DataTable is serializable in Net 2. Remember that a TypedList inherits from DataTable. What you can do is add the TypedList to a DataSet, return the Dataset from the WebService and take the TypedList from the returned DataSet.
Edit.
I see that you are using VS2005, then you are using Net 2. In this case if you want to return the TypedList directly you need to modify the generate wsdl class of your webservice (I do not remember the details right now). To avoid be changing the wsdl class I would go with option of adding the typedlist to a dataset and returning the dataset.