As you can read on the iPhone support pages, Tamil isn't supported (yet) by the iPhone:
English (U.S.), English (UK), Chinese - Simplified
(Handwriting, Pinyin, Stroke), Chinese - Traditional (Handwriting,
Pinyin, Zhuyin, Cangjie, Stroke), French, French (Canadian), French
(Switzerland), German (Germany), German (Switzerland), Italian,
Japanese (Romaji, Kana), Korean, Spanish, Arabic, Bulgarian, Catalan,
Cherokee, Croatian, Czech, Danish, Dutch, Emoji, Estonian, Finnish,
Flemish, Greek, Hawaiian, Hebrew, Hindi, Hungarian, Icelandic,
Indonesian, Latvian, Lithuanian, Macedonian, Malay, Norwegian, Polish,
Portuguese, Portuguese (Brazil), Romanian, Russian, Serbian
(Cyrillic/Latin), Slovak, Swedish, Thai, Tibetan, Turkish, Ukrainian,
Although, iOS5 is capable of displaying Unicode Tamil email and text.
But you'll have to use 3th party apps like IndicNotes which will allow you to type Tamil. Then you can share your note with SMS, Email,... Not the most ideal solution, but you'll be able to type Tamil on your iPhone.