hi,
i have done something like this in the past. basically we gave the usere a link to download their report data as a csv file.
basically you just provide a hyperlink to the file eg..
<a href="data.csv">
you then configure the mime type for csv to be application/octet-stream
this means that when you cloick on the link, your browser will prompt the user to save the file.
if you have a dynamic file such as some repor tdata for that particular user, you could have the link go to a servelt, that could create the fiel and then redirect (i'm not sure about forward) the request onto the newly created csv file.
hope this helps - dean