Hi Jay,
If you are using the UpdateListItems Method of the Lists Web Service as suggested in Matt's Post, the GUID of the list being updated is stored in the ListName field as a default value (remove the curly braces {...}, so that there is just the number and hyphens "38472564-2......73634A5" ). This is the GUID if the list only, not any individual columns.
The CAML Template will need to have an XML Field element to map to each field in the SharePoint list which you are updating. Once you have populated the CAML Template with the required data to be submitted to the SharePoint list, the Secondary Submit data connection will Submit the CAML template using the Lists Web Service and the UpdateListItems method.
You will need to make sure you are using the "internal" columnn names when mapping to fields using the CAML template. The U2U CAML Query Builder can assist with finding the column names to use in the CAML template.
When configuring the submit data connection, selecting "XML Subtree, Including Selected Element"is an important step, but this should only be done for the "updates" paramater. Leave the default option for the ListName paramater, so that the value inside the field is used, not the field as an XML element.
Another article which may assist explains how to update existing items in a SharePoint list using a similar technique, with some important notes for data types when updating data in SharePoint using web services.
See InfoPath - Update Existing SharePoint List Item Programmatically Using CAML for details.
Thanks
Daniel