Getting Up and Running with Excel Services
Configure Excel Services so that you can publish an Excel spreadsheet to a Windows SharePoint Services 3.0 site.
Taking this a step further (wondering aloud really), if you have a spreadsheet of sales figures with tabs representing regions, and all data rolling up into a dashboard on the first tab, then you can conceivably render in the browser only the first tab through Excel Services, while those responsible for feeding the data have secure access to the additional tabs in the Excel document in the SharePoint library. Said library can include check in/check out and version control to make ongoing maintenance a snap.