EMail as Primary Key in List

Problem:
I have been asked a question today from one of the developers in another team that he wants to create a user registration list where emial should be unique for every user. And if user add item with existing email then he should be informed.

Solution:
We know that this functionality does not exist in the OTB SharePoint list features. After googling a bit I found following excellent solution using JQuery:

URL: http://sharepointjavascript.wordpress.com/2009/11/23/sharepoint-lists-or-document-librarys-primary-key-in-selected-field/