(in via Martin)
To the observant reader of Microsofts own Lifecycle policy it is not new that the file format for Excel 2003 (xls) is about to be de-continued with the end of extended support for Office 2003 in April 2014
(see 
http://support.microsoft.com/lifecycle/default.aspx?LN=en-us&x=13&y=10&p1=2488)
Hence, it is only reasonable that OBIEE also discontinues the Office 2003 products and formats after a certain grace period.
To quote the announcement made in 
KM Doc - Support of Microsoft Office 2003 with Oracle Business Intelligence Answers, Dashboards and Publisher OBIEE 11g [ID 1558215.1]:
"Over the past several releases, Oracle Business Intelligence has supported Office 2007, Office 2010 and the newer Office XML file formats (*.xlsx, pptx, *.docx). Customers are strongly encouraged to use the Office XML formats and the Microsoft Office versions that support those formats. Starting with Oracle Business Intelligence 11.1.1.8, Oracle will begin removing functionality that supports Office 2003 products and file formats."
Having said that, a closer look at the attached PDF shows that not all hope is lost on your old spreadsheets. Some functionality will be carried over and continued to be supported for some time. e.g:
- Excel Templates will continue to support *.xls as the template type and the output type
- RTF Templates and XPT templates will continue to support *.rtf output
- Excel binary *.xls files will continue to be supported as a data source type
Reassuringly this means, that you still have time to adjust your projects and processes and get used to using xlsX where possible.
So far, we did not get an announcement when the xls format will be finally buried, but thinking ahead it is likely a wise move to start using xlsX as your new standard.