Marcos,
nesses links abaixo tem a sua resposta, depende do cenário que vc esteja utilizando.
http://forums.asp.net/t/1816008.aspx/1
http://stackoverflow.com/questions/262341/get-data-from-an-uploaded-excel-file-without-saving-to-file-system
http://www.dotnetfunda.com/articles/article706-how-to-read-csv-file-and-import-its-data-with-linq.aspx
Abs
Rogério Rodrigues, Microsoft Student Partner, Desenvolvedor .NET, CSD, MCTS TFS 2010