I don't think there is an SPS web service to do this, but you could build a
web service yourself. There are two ways you could retrieve the XML data for
an InfoPath form: 1. Use System.Net.WebClient and pass the URL of the form,
or 2. Use the SPS object model to get a file from a library of an SPWeb.
Search the microsoft.public.sharepoint.portalserver newsgroup on
"Programmatically download a document from Sharepoint" for code samples on
how to do this. You download a form in the same way you download a document.
Instead of downloading the form, you would have to read it into a Stream and
then read the XML data from this Stream.
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S.Y.M. Wong-A-Ton