Our organization is finally deploying Office 2010 to all our employees and as part of that deployment is compatibility testing with our current applications. Our crime analysts came by an interesting problem where ArcGIS 10 was having issues opening up an Excel 2010 file (xlsx).
The culprit of this issue was the fact that the computers that were running ArcGIS did not have the latest Microsoft Office Data Connectivity components installed. Depending on your organization and how you serve your apps, you may not want to have Office installed on the computer/server that is running ArcGIS, hence this issue may arise.
To solve the issue, just download the components from http://www.microsoft.com/en-us/download/details.aspx?id=23734 and install the drivers. (I tried using the Office 2010 Connectivity objects and it didn’t work, hence the link to the 2007 components).