it's been mentioned and mentioned and mentioned again. unfortunately apple don't seem bothered about adding it. probably because it would ruin the "feel" of it being like a chat app rather than SMS. i send a lot of texts and found that i was going over the limit because you view it as a chat app and start treating it as one. i just cut my tariff down from the £35 a month tariff to £30 and added the unlimited text bolt on as i don't use many minutes.
try submitting it as a feature request if they are still accepting those. that's what i did when i first got an iphone. was on the apple website somewhere, google it.
http://www.apple.com/feedback/iphone.html