Reporting back some solutions:
For Android users, I had them switch from QuickOffice to KingSoft Office which is free and works great, even for complex, multisheet XL files generated by RS by RS 2008 R2.
For Pc users of Excel 2010: Our users were getting an error (some data has been lost) when opening an Excel file generated by RS 2008 R2. After clearing the dialog box, it would appear ok. I solved this by applying service pack 2 and cumulative updates through #5. I was hoping that would solve the problem for the Iphone users.
For Iphone, users, I'm still drawing a blank. No free apps are working. Some do open the file, but the formatting is messed up. Some can't open the file period.
So, if there is a Iphone app (free or not) that can handle complex, multisheet XL files generated by RS 2008 R2, please let me know.