Borrowing from Flex/Rails libraries created about a year ago, I have created a Flex/Grails application using similar HTTP/XML protocols. The application requests report data via HTTP, the server creates the report, persisting it to a file, then the client pulls the data back with a file request. This was the only way I could get Flex/Flash to open a "file-save" dialog to let the user write to their local hard-drive.
I used Flex's HTTPService object to do the initial report request, then FileReference with additional URLVariables parameters to retrieve the file. Grails/GORM made the data retrieval easy and the controller/service packages were simple implementations. This was comparable to Flex/Rails implementation that I worked on a year ago.
The server side runs in a war distribution so the output file had to exist outside the webspace. The Flex application requests the file with an HTTP post using URLRequest with specific parameters (URLVariables) to retrieve the file. Because the server is deployed with only war files, the request is made to a specific controller with a "getfile" action that retrieves the file from someplace on the server's drive (obviously not in the war file) and is consistent with where the file was originally written. Once the file is returned to the client it is removed, so it can only be read back a single time.
This first iteration returns a CSV file, but creating a full xls spreadsheet or a PDF document would be easy to implement. There is also an xml document that can be returned to enable showing the report in a local viewer, e.g., a data grid.