If you use DataSet.GetXML, it returns the Dataset in the form of a XML string. Using Dataset.ReadXml will populate a Dataset from an XML string (or a stream).
Lastly, your premise for the tip itself is flawed. Ninety nine times out of 100, a report is indeed SUPPOSED to query the database. What you're doing is storing what amounts to a snapshot of the database, duplicating the data, and thus unnecessarily consuming the user's disk space (or worse, the server's disk space).
Of course, I'm not familiar with your requirements, but a properly formed query will always generate the same report, so there's really no point saving the report data to a disk file.
Just sayin...
