If you use Isapi_Rewrite for IIS, there is a very clean solution to using "inline" and getting the correct file name for the Save As... Dialog.
Instead of making the link to ReportStart.aspx, make it to a fictional Directory like
http://mysite/Download Reports/xx/MyFileNameAsIWouldLikeToSeeIt.xls where xx is the information ReportStart needs to be passed as a parameter.
In your httpd.ini for for isapi_rewrite, add a RewriteRule:
RewriteRule ([^.?]*?/)Download Reports/(.*)/(.*) $1ReportStart.aspx?id=$2 [I]
For those of you who don't live and breath Regex:
http://mysite/Download Reports/xx/MyFileNameAsIWouldLikeToSeeIt.xls
gets turned into:
http://mysite/ReportStart.aspx?id=xx
The document should open in your browser. But the URL remains...Click Save As...and the file name is MyFileNameAsIWouldLikeToSeeIt.xls