@Dan: XLSREAD has a bug in its implementation to read xlsx files; I have informed Mathworks support regarding this. The Mathworks development team has acknowlodged the bug. Don't know yet if they fixed it.
The workaround is to use xls files if you can.
A great submission but there seems to be problems when I use xlsread to read these files. I am unable to read an xlsx workbook with multiple worksheets created with xlwrite (on a Mac). I just get "Worksheet not found".