You must have gone through my previous article on Oracle Index rebuild – 10g Vs 11g – https://avdeo.com/2011/03/17/oracle-index-rebuild-online-10g-vs-11g/
Well, there is another nice article written by a friend of mine Sumit Bhatia.
He has talked about the actions and events happening while rebuilding an index online in oracle database 11g.
Different wait events that we encounter and to make it more practical he has also build a script which gives you the exact status for your index rebuild.
You can refer to the article at http://fordba.wordpress.com/2011/04/05/online-ndx-rebuild/
You can refer to the script at http://fordba.wordpress.com/2011/03/30/ndx-progress-sql/
Just to mention, this script is by no means a complete script and enhancements can be underway. Also this script should not be used in production. If you want to use in production, you can do it at your own risk.
Hope this helps !!