If you are using APEX Listener version 2.0, you can download a sample application here. In this application I am showing how to:
1. create an excel upload page
2. manage multiple excel sheets after upload
This new APEX Listener feature is great and I hope they will extend it to the other excel file formats - currently it works for .xls only. If you are using APEX 4.2.3 this will probably not work since there is a bug in that version of APEX.
So, the current limitations are:
1. xls files only
2. APEX 4.2.3 is buggy related to this feature
3. you can upload up to 49 columns since it is using APEX Collection and one column is used for excel tab names during the upload