Ed Cones answer serves to allow you to create Excel files that may be used by
people with earlier versions of Excel when you don't know specifically what
version they have. Good answer.
There is also a tool (a 'Compatibility Pack') available from Microsoft that
you can install on systems with earlier versions of Excel (and/or Office)
that will allow earlier versions of Word, Excel and PowerPoint to read and
work with those types of files created with Office 2007 applications.
Instructions and package are available at:
http://support.microsoft.com/kb/923505