Since I am working on localization a lot, many people ask me that how to type in Tamil. I wrote a small passage on that published that on Joomla Tamil forum.
You can find that here : http://forum.joomla.org/viewtopic.php?f=545&t=349037
With that note I also have attached few other document that I have prepared on this.
N.B. The given link will direct u all to a forum in Joomla.org. I am the moderator of that forum :)